Casually parked outside the imperious Monte Carlo Casino sit a couple of Ferrari 575s, a Porsche 911 Turbo and a Maybach. In the last hour or so we've witnessed a graphite grey Murci΃©lago amble past, dozens of gleaming 360s bark down the narrow streets and even a 550 Barchetta heading for the hills above the principality. It seems that photographer Andy Morgan, road test hero John Hayman and I are the only ones to notice. Exotica are simply invisible to the residents of Monaco. It takes something new to snap them out of their well-rehearsed indifference, and we've got the keys to the hottest car in town.

BMW's new 645Ci literally stops the traffic in Monaco. The immaculately turned-out Casino doorman is happy for us to park up outside the regal frontage for as long as we want, locals wander over to compliment us on the car and at one point I'm even followed by an excitable German who has a 645Ci on order. 'Beautiful' and a huge smile is all that most can muster as they take in the lines of the latest 6-Series. And what of the lithe Maserati Coupe parked alongside? It doesn't even register. Not a word from anyone. Whether the fascination with the new Six will last when it becomes a more common sight is open to question. Right now, though, as far as the Monaco-set is concerned, the 645Ci is the car to be seen in.

At a basic ΂£49,855 it undercuts the Maser by a substantial margin, and remember it wasn't so long ago that the Italian coupe seemed like a genuine bargain: the promise of near-911 Turbo pace coupled with svelte lines and an innate charisma that only a Maserati can generate at a mere ΂£60,000 seemed too good to be true. And it was. Despite a glorious engine and a rich character, the dynamic prowess of the Maser fell short of the promise.

Recently, though, there have been signs that the Coupe is finally shaping up to be the car we always wanted it to be. It's hard to keep track of all the various refinements, but with every passing month the Coupe seems to get better and better. Our Italian-registered Argento Luna example is well used with over 25,000 kilometres and a patina to match, but it has the latest software for its Cambiocorsa paddle-shift 'box and new-for-2004 settings for the Skyhook suspension.

So we have two cars with one aim: to be the consummate sporting GT, and one journey to discover their true abilities. Two days and 800 miles on a combination of tight mountain passes and wide, fast, French N-roads strung together by lightly trafficked autoroute and finally the grim reality of the British M-road system. It's a real test of the cars and it should provide an equally telling insight into the relevance of the GT in a world of low-cost airlines. Can either provide the spine-tingling thrills of a proper sports car up in the mountains yet still provide hushed and luxurious accommodation on the mundane major roads that are a necessary evil on any continental trip? And even if they can, would you really rather spend two solid days slogging across Europe, even in cars as special as these, when a two-hour flight at under ΂£20 is just a mouse-click away?

The 645Ci might be a hit in Monaco, but the evo budget can't stretch to a night in the Hotel du Paris so once the photography is in the bag we head along the coastal road towards Nice in search of more affordable accommodation. The BMW's superb DVD-driven sat-nav makes this a doddle. Simply type in 'Nice' and then scroll along to 'Information on Destination' and a list of 20 or more Michelin-recommended hotels flashes on to the 6.5-inch dash-mounted screen. If the phone were connected it would even dial the hotel to allow us to make a reservation. It's a mark of the attention to detail that permeates the whole 6-Series package and on the chaotic descent to Nice centre it's an enormous boon.

Our Titanium Silver Six does without the SMG paddle-shift 'box (an ΂£880 option) or the six-speed Steptronic auto (΂£1350) but the standard six-speed manual is excellent: smooth shifting and positive. Considering that the 4.4-litre V8 kicks out 328bhp and 332lb ft, the clutch is incredibly friendly, too. A Cambiocorsa-equipped Maser should be even better in stop-start town driving like this, but I can't imagine you'd ever curse sticking with the manual 'box on a Six.

But then the whole driving environment in the Six is impressive. The optional sports seats, which move in every conceivable direction, and the electrically operated adjustable steering column (both reach and rake) allow a finely tailored driving position, and everything you touch feels solid, expensive. Even the new-generation iDrive seems clear and easy to operate. Only the view out of the windscreen disappoints. Visibility is fine, but there's no sense that you're in an extravagantly styled coupe, no dramatic swoop to the bonnet or proud wings to frame the road ahead. It's a small detail but it's important that a car like this never feels ordinary. After all, you can always buy a 5-Series...

With the cars safely tucked away in the recesses of a gloomy underground car park, we head into the old town district of Nice, looking for sustenance. After the oddly sterile streets of Monaco, the charm and character of the Cours Saleya, lined with fresh fish restaurants and stained with decades of over exuberance is like a breath of fresh air. Even the Maser seemed happier to leave the fickle world of Monaco behind. As we threaded through the busy streets en route to the hotel, it finally got its share of the limelight as pedestrians looked on in quiet appreciation.

Hayman, however, fears the Maser may have been noticed for the wrong reasons. It seems that the Cambiocorsa 'box isn't the smoothest and heads were probably turned as much by the soaring engine note and jerky progress as the fluid lines. Morgan, who was in the passenger seat, concurs. Oh dear. I've yet to experience a Maserati Coupe and desperately want this to be a good one. Tomorrow will be a big test. A quick blast along the A8 to Grasse and then we'll be climbing the soaring N85 (we know it as the Route Napoleon) all the way to Grenoble, nestled deep within the Alps. With faster and less testing roads once we clear the spectacular mountains, I can't help feeling that if the Maser can't shine on the Route Napoleon it's game over.

Nice is already bathed in the cool light of the winter sun as we leave our hotel on the Wednesday morning. It's 8.30, there are pictures to be taken, and we need to make a massive stride into the heart of France if we hope to get home by Thursday evening. I've got the keys to the Maserati and I'm a bundle of nerves. For me the Maser is quite simply one of the coolest, most desirable cars at any price. I love the name, the fascinating chequered history, and the dramatic Giugiaro lines... but I've never driven one. And I don't want the spell to be broken.

In the dim yellow light of the car park the Coupe looks divine, especially next to the Six, which manages to appear bulky and a bit heavy-handed. It towers over the svelte Maser. I plip the key, pull the slightly flimsy door handle and drop into the dazzling cr΃¨me leather-lined interior. The seats are noticeably narrower than those of the Beemer, the footwell cramped even with just two pedals, but there's a warm, feelgood factor that the brutally functional Six would never dream of trying to emulate. The steering wheel adjusts for both reach and rake but the manual release is painfully insubstantial. Still, don't dwell on the negatives, revel in the mini-Ferrari 575 ambience, and it's hard not to be won over.

Twist the key and the Maser bursts into life with a big-hearted howl, settling to a fast idle that gently shakes the car on its springs. Pull the leather-trimmed right-hand paddle and we're away. Nice is still bustling and the Maser feels rather ill-at-ease as we crawl along the Promenade des Anglais, crashing into bumps that the 645Ci would smother and, even in auto mode, failing to shift decisively or with any degree of finesse. The 4.2-litre quad-cam V8 immediately feels ultra-responsive, though, as you'd expect with 390bhp and 333lb ft on tap.

Finally we clear the A8, negotiate Grasse and head up onto the N85. It's hard to believe how quickly the broken urban surroundings vanish and are transformed into wide, pristine tarmac that cuts its way upwards through the scarred pale rocks to deserted towns set amidst breathtaking scenery. This is a seriously fast road, but with intimidating drops often flashing past the side window it pays to show it some respect. The Maser, free from the constraints of jostling traffic, finally gets to stretch its legs.

The promise of the muscular engine at low speeds translates into epic pace when you get the chance to really wind it up. Like the BMW, it will pull hard from a little over 2000rpm, but the Italian car has a thirst for revs that spurs you on to the 7500rpm redline. It rewards with a transcendent explosion of power and noise at the top end, and I find myself closing rapidly on the Six up ahead.

Through fast curves the Maser settles on its springs at quite an angle, but the roll is stable and the chassis gives you clear signs of how much grip is left. The steering is ultra quick and oddly weighted, but the car stays planted and you can really start to lean on the outside tyres. Too much commitment and things get a little messy, though. The roll becomes ever more dramatic, which leaves very little suspension compliance for mid-corner bumps and can result in the big coupe lurching uncomfortably off line. As the Route Napoleon gets more nadgety, and as I find myself needing to make more steering inputs and more frequent gearchanges, slowly the 645Ci begins to re-establish the gap.

Morgan has spotted another photography opportunity and we stop for a welcome breather. The furious ticking of hot metal emanating from both the Maser and the big Six betrays their efforts on roads like these. Even with extensive use of aluminium, magnesium and Thermo-plastic, the 645Ci weighs 1615kg, the petite but less cutting-edge Maser 1680kg.

Hayman has been enjoying the Beemer, but the Route Napoleon can expose any chink in a car's armour: 'It's a really nice thing; great engine, a really sweet 'box - and I was comfortable with the Active Steering straight away - but it's just a bit too soft for my liking,' he says. I have similarly mixed feelings about the Maser. Despite the hyper steering response, the thumping paddle-shift and the searing power it doesn't quite gel into a driver-focussed package on a road as demanding as this. It's like an over-eager puppy, the Coupe: always desperate to please - all the controls react without the tiniest hint of slack - but there's little subtlety and the fairly lax body control is at odds with the uncompromising nature of the drivetrain and steering.

Hopping straight into the Six, it takes only a few hundred yards to discover where it's finding that pace. BMW's sophisticated Valvetronic 4.4-litre V8 may be freakishly silken and running at half volume compared with the singing Maser, but it packs a mighty punch and seems perfectly tuned for the N85. It finds neck-snapping pull out of the slower corners but does its best work between 80 and 120mph - speeds that are easily attainable on the quicker sections of the Route Napoleon. The Dynamic Drive system fitted to this 645Ci (at a cost of ΂£1755) helps, too. Essentially active anti-roll bars with electronic pressure control valves, the system cuts body-roll by up to 80 per cent under cornering forces of up to 0.6g and keeps the broad Bridgestone run-flats pushed firmly into the road.

The Active Steering (a variable-ratio rack that's super-direct at low speeds but requires more input as speed increases, theoretically to aid stability) should be a recipe for lifelessness, but in fact it relays more detail than the Maserati's traditional rack, loading up nicely just when you need something to nudge up against. Hairpins are rounded with just a flick of the wrists, but at speed there's no nervousness. However, it does take a bit of time to recalibrate your senses; especially if you've just darted through a tight turn and then approach a sweeper expecting the same rate of response. Ideally, the steering would be a bit quicker at speed.

Even so, it's incredible that so much technology can be integrated into a car that feels so organic. The chassis is resolutely neutral, only pushing into a slight understeer on total-commitment, clear-sighted corners. Turn off the DSC and you can enjoy a bit of nicely telegraphed oversteer, but it's more likely to smoke its rear tyres than flick sideways, in the dry at least. If you want the safety net of stability control without the killjoy effects of total traction control, a quick press of the DSC button will disable the latter but help out if things start to get out of hand. You're only left entirely to your own devices with a determined five-second prod of the DSC button.

I stay in the BMW as the N85 wends its way back down towards sea level. Despite the fact that it never entirely shrugs off its weight (actually fairly modest in this class) and the feeling that it's even more at home as the conditions get less frenetic, I'm impressed that it's coping so well. The danger with any GT is that you'll make a huge effort to get to the sort of roads that define a truly memorable trip and then find that the thrills you seek aren't there for the taking. Both the BMW and Maser can provide moments of real excitement, but round 1 goes to the 6-Series, rather against expectations.

The Alps tower over Grenoble, sprinkled with snow. I'm back in the Maserati now and, after the challenges of the Route Napoleon, all we really want to do now is get to our next destination. The A48 to Lyon is quite a narrow, twisting peage but it's the quickest way north, the photography is over for the day and we're all getting hungry. It's rush hour but the French traffic moves freely. Even so, Hayman slices past the locals in the Six and we settle into a 110mph cruise.

The Maser is busy at this sort of speed - at an indicated 180kph (112mph) the big V8 is spinning at over 4500rpm. You need to raise your voice a decibel or two to hold a conversation, and the tyre-roar and wind hissing around the A-pillar can become a bit irksome. The trade-off for the industrious engine note is superb response, though. Every time a dawdler strays into the outside lane slowing our progress, a whiff of throttle soon restores those lost mph. As Lyon approaches we veer onto the A6 and press on to Macon. Despite a 12-hour day, with the stops for photography we've only covered around 300 miles and left ourselves with a good 400-mile journey just to get to Calais tomorrow.

Next morning it's still dark and the roads are covered with an ominously frosty sheen as we nose out of the grand-looking Hotel D'Europe et D'Angleterre, which, although once a haunt of the likes of Queen Victoria is now a youth hostel in disguise. I'm back in the big BMW, simply because we're returning to the A6 autoroute before joining the N71 near Dijon and I've yet to do any meaningful motorway miles in the Six.

After the frantic Maser the 645Ci is an oasis of calm, and in these nasty conditions its 245/45 section Bridgestone cut cleanly into the surface. The Maser has a big wobble in the icy conditions before we've even left Macon, and as we join the A6 it soon becomes a tiny spec in my mirrors. The BMW seems impervious to adverse weather conditions and soon I'm enjoying a hushed 100mph cruise. The rev-counter shows 4000rpm, but the V8 is so well mannered that it might just as well be 1500. Only the rustle of wind noise around the A-pillars distracts, but I'm surprised to note a marked susceptibility to crosswinds.

A petrol stop allows the Maser to catch up, but Morgan looks a little ashen-faced. 'The traction control didn't seem to wake up as early as we did this morning,' he grimaces. 'Feeling the thing move around at 100mph is a bit scary.' Even John Hayman in the passenger seat looks a little grey, which must have taken some doing. The Maser was supplied on directional Michelin Pilot Sports, a tyre that has recently been replaced with a much-improved asymmetric design. That might account for the instability on the slick surface, but the sleepy MSP (Maserati Stability Program) is more of a worry.

I decide to stick with the 6-Series and let the sat-nav lead us onto the outrageously fast N71 that will take us all the way to St Quentin, a stone's throw from Calais. It's a fabulous road, the perfect place to really understand what it is that makes a great GT car. Mile-long straights are linked by combinations of 100mph sweepers, then a village might slow things down before the road opens out again and twists through a patch of woodland, only to emerge onto another seemingly endless and tempting straight.

The 645Ci is in its element, blitzing the faster sections and pouring itself through the more testing third- and fourth-gear corners with no fuss but enough involvement to keep me alert and buzzing. The Maser has the measure of the BMW in terms of sheer animalistic urge but I'd wager that the Six is quicker on this road. It's so stable and surefooted that it builds your confidence until you're nibbling away at the limits of grip even on very fast corners. It's inspiring stuff and confirms what I'd suspected since the Route Napoleon rose majestically up from Grasse. The GT isn't dead; in fact it's in the rudest health.

As the Maserati skims over crests behind me, loading up on its springs through the endless arcs of tarmac, I know that Hayman and Morgan are having fun, too. The Italian car may lack the little things that make the 6-series the consummate all-rounder, and it has a few rough edges still to be ironed-out, but it has a unique charm. Is that enough? Probably not with rivals as fearsomely advanced and beautifully resolved as the 6-Series around, but it still tugs at the emotions like few cars we know.

Soon the N71 crosses the A26 and we're back on the autoroute for the final blast to Calais and the Tunnel. It's a risky business pinning the throttle against the carpet - the gendarmes like nothing better than collaring a Brit taking his last chance to reach big speeds - but I haven't seen 150mph on this journey and I'm back in the Maser...

No need to change down, just nail it from around 100mph in sixth, the nose points skyward, the engine note takes on a new menace and the speedo starts its relentless haul towards the big numbers. Hayman has already checked to see if the 645Ci has a limiter (it does at an indicated 160mph) but the Maser is happy to go further still. At a rock solid 270kph (168mph) and close to 7000rpm I back off. There's more to come but my sense of self-preservation kicks in and I start scanning the horizon for radar traps, helicopters, anything that looks remotely suspicious. Ten minutes later I'm still a free man, with another incredible sequence burned into my memory.

Rolling out of the train in Britain, under the cover of darkness and a blanket of drizzle, I'm glad I took my chance while I could. The M20 is moving freely enough, but even 80mph is out of reach at the tail-end of rush hour. The Maser is less frenetic at the reduced cruising speed, the Six barely ticking over, and just like back in Monaco it's the Beemer that's turning people's heads. Say what you will about Bangle, he knows how to make an impression.

The M20 morphs into the M25 and soon we're heading up the M11 towards Stansted, where Hayman and Morgan's cars are waiting. The trip is nearly over, the cars are grimy and littered with empty bottles and crisp packets, but visions of the Route Napoleon and the snow-peaked Alps still fire our imagination and our affection for both the 645Ci and the Maser has swelled enormously. While we're unloading Morgan's camera gear from the BMW, an airport bus pulls up and the weary, short-tempered passengers trudge through the car park, doubtless dreading the long drive home. The final leg of my journey will be a bit of a drag, too, but at least I haven't got the aftertaste of instant coffee and a ΂£2 chocolate bar reminding me of a long wait in a packed departure lounge and a painfully cramped flight home. Queasyjet or GT? No contest, really.