It was a wholly surreal experience. The Isle is so obviously British - it's a kind of cross between Lancashire and Northern Ireland scenically, culturally and architecturally, as befits its location - but it isn't directly a part of the UK or even the EU. So it can do things its own way, and that includes many limit-free roads which have the effect of making people stick closely to the limits where they do exist. Safety zealots take note.

For the record, I didn't get beyond 105mph thanks to bends and traffic. The other two were luckier, and maybe braver. But ultimate speed wasn't the only point. We were there for freedom, for the joy of driving two great cars quickly and safely without speed cameras, the echoing chorus of speed-hating hand-wringers, or the fear of being branded criminals for victimless crimes. Think of it as the last open road.

The Isle of Man and the TT motorbike races are welded together in most motor-minded people's heads. Somehow, the top riders manage to average 124mph around the 37.7-mile circuit that's made up of closed public roads. Given that many of the roads pass through towns and villages, have narrow twists and hairpin bends, and are littered with street furniture and unfriendly solid objects, that seems a scarcely believable pace. The fastest speed we reached was barely higher than the Senior TT racers' average. Even in the imagination it defies belief. When you've seen the roads for real, it's even stranger.

Our two chosen cars each have a point to prove. One is the faster, torquier, less unluxurious Lotus Elise they call the 111S, its mission to prove you can mix Eliseness - minimal weight, fabulous sharpness, total tactile transparency - with the sort of sophistication that might tempt a Boxster owner away from the security of Porscheland. The other is the latest incarnation of that most affordable Porsche, the revised Boxster with subtle visual tweaks and a dash of extra power. Much admired for its keen, faithful handling and smooth pace, the 'basic' Porsche could attract Elise owners craving something a little less hardcore. Somewhere, the appeal of each car overlaps. The question is, where?

Both are mid-engined. Both have a boot behind the engine, rather bigger in the Boxster which also has cargo space up front. But even the Elise has more of a boot than a Toyota MR2. The Porsche has more power (228bhp against 156), the Lotus has a better power-to-weight ratio (199bhp per ton against 182). Both even have air-con, a ΂£1295/15kg option in the Lotus. There is, however, a more fundamental difference. The Boxster seems to tower above the Elise as they meet at Liverpool dock; where the Lotus emits lightness, lowness and litheness, the Porsche is all solidity and substance.

Truth is, though, after a rapid run up the M1, M6 and M62 to Liverpool, I'm already a bit off the Elise. The concrete surfaces are the worst, droning and thudding through the bonded-aluminium structure via suspension joints whose lack of give is the Lotus's fortune and failing in equal measure, context-dependent. The driving seat is hard, too, especially the backrest. This is not a car in which to keep your body static.

Then I get lost in Liverpudlian suburbs as I search in vain for meaningful signposting, and end up going through the Mersey tunnel twice and narrowly missing a motorcyclist because I didn't realise the rear blind spot was so big. This is not the Lotus's ideal habitat. I bet the boys in the Boxster aren't suffering like I am, and I've always been one of the Elise's biggest fans. Then I scrape the bottom of the front valance as I drive onto the Seacat's entry ramp. Damn.

Anyway, we arrive in Douglas after a mercifully smooth crossing, and find our econoclass hotel. Sea View, it's called. Sea Glimpse more like. There's an old car rally on, so the promenade is sprinkled with tasty machinery - a Ferrari 250GT Lusso heads the desirability list - but there's little sign of the expected superbikers.

It's morning, and we're going to drive the TT course; the full mountain route, not the little sprint course which scythes through the unlikely setting of Peel's housing estates. I'm in the Boxster, and straight away it's a different world. The door has closed with a weighty thunk, every control feels smooth and damped, the seats are snug and yielding, the wheel seems enormous. Then I turn the key, to be greeted not with timing belt whine and cold tappet clatter but with the deep, smooth-but-beaty burble of a fine flat six. Porsche's first watercooled six-pot doesn't have the aural edge of an old-school 911, but the lineage is still sonically unmistakable.

This is very civilised. The lever slides easily, accurately into first (the Lotus's linkage feels springy, loose and full of misplaced ball bearings by comparison, for all its economy of movement), and the first-to-second shift, always a measure of a car's driveline togetherness when enacted for the first time, is completely fluid. I like this car already. Maybe I'm going soft.

We smooth up to the TT circuit's start, in the middle of Douglas next to the cemetery and the police headquarters where three worlds collide. Race position boards, pits, paddock, it's all here, all permanent. It's hard to imagine this busy but 30-limited street hosting a phalanx of fierce racing bikes which will pile through the first set of traffic lights and downhill to the double roundabout, but that is what they do.

Following the course is easy; if there are kerbstones they are painted alternately black and white, and large orange signs point the way, identify 'milestones' and count down to hazards. Other clues abound, too. Sharp, solid objects are likely to be barricaded with big bags of soft substances, in the process of being taken away during our foray, and from time to time a bunch of flowers appears by the roadside. Freedom has its price.

The first part of the course is speed-limited now, with 50 or 60mph signs newly wired to lamp-posts even on the open road. That's fair enough; the roads are busy and full of hazards, and going much faster would not be wise. In fact, when you turn off the westward road to Peel and head north-east towards Kirk Michael, the twists and swoops are exciting enough even at 60.

After Kirk Michael, though, the limit is gone, the road opens out and the Boxster is given its head. But this is a small island, nowhere is very far from anywhere else, and there's no room yet to get a speed-rhythm going. And then we come to Ballaugh Bridge, a hump-back after which the airborne riders must force a right to avoid slithering into the pub car park. Past the half-way point we come to Ramsey, on the far side of which is a hairpin left where Honda corralled a gaggle of Civic Type Rs on the launch a year ago. That time, we set off one by one after a police Accord Type R had hurtled off ahead to check the mountain road was closed. We could go as fast as we liked, even in the rain and fog. This time, there'll be no such escort.

Yes, I remember this. After the hairpin, two straight blasts separated by a fast right, then a fantastic sequence of uphill twists climbing towards Snaefell. It's damp under tyre, and just for fun I apply a power excess as we exit a tight corner. Out steps the back; I come off the power and steer into the slide, not expecting it to decay so quickly as the Boxster gathers itself together. Which means I've over-corrected, and we're now heading for the bank before I uncorrect again. The moral: trust the Porsche, and don't try to be clever. Not sure I like that.

A couple more things I'm not so keen on, after a day spent in the Elise. Why are the Boxster brakes so unprogressive, snatching like an over-servoed Audi A4's? And why does the steering feel edgy, unlinear and remote all at the same time? A little turning effort has a big result, yet you can't feel the consequences through the wheel's rim. Then I realise. Normal standards have been skewed because we're in the Elise's presence. In isolation, the Boxster is a paragon. It just turns out that it, too, is more of a compromise than I thought.

There's not much wrong with its pace. This is the regular, 2.7-litre, 228bhp Boxster and not the 3.2-litre S version, but it pulls with honey sweetness and a broadening blare from that extraordinary centre tailpipe right up to 7200rpm. The road stretches ahead, fast bends calling for commitment, but what happens on the edge of the grip abyss is a little less clear than I'd like. Then we hit the fog. Lots of it, rolling in waves across the mountains, and that's that. Time for a quick look around the TT museum further along the road before the tramway crossing - there's a new memorial there to commemorate multiple TT winner Joey Dunlop, who met his end hitting a tree in Estonia - and on to the pub at Creg Ny Baa, on possibly the TT's most terrifying corner. It comes after a downhill switchback, the fastest part of the circuit.

The next day dawns free of fog and other moisture. I've been missing the Elise - I've always liked cars with extremes of character, as long as the good outweighs the bad - and we're going to drive the course again. Off comes the Lotus's roof, an easy task on the latest cars as you unclip the side rails, roll up the structure and unspring two supports, and down disappears the Porsche's electric soft-top into its metal cover.

This is how the Elise should be. I can hear the exhaust better now, hear how it turns into a straight-through system above 4000rpm and three-quarters throttle, and the booms and rattles can dissipate into outside air. The 111S (there's also a straight 111 with starker trim) uses the VVC version of Rover's 1.8-litre K-series engine, which varies the opening duration of the valves as well as their initial actuation point. Power is the same as in the MGs so equipped, but Lotus has its own management system and is proud that the 111S is in the lowest CO2 tax band. Behold the eco-friendly sports car.
Even when ambling, the Elise feels perky. You can be at 2000rpm in fifth gear, a highish ratio for relative motorway bearability, and it will pull hard if asked. At first I thought the 111S was little quicker than a regular Elise, but the feeling is deceptive; there's simply more of everything right through the rev range, rather than a sudden surge of extra energy.

Now we're out of the Ramsey hairpin and powering up to the mountains, and the Elise is in its element. The steering is transmitting every topographical nuance to a degree that some might find irritating but I just love. The unservoed brakes need a hard push but are breathtaking in their firmness and progression, like a racing car's. The balance is perfect, the grip on the sticky, short-lived Bridgestones is huge but signals its limit in good time. This is a car you can drive right up to its edge, or yours, because the running commentary is so detailed. I'm sitting almost on the road in a car of seemingly no inertia, and it's just brilliant. Even the daunting downhill right-hander at Windy Corner, medical hut stationed on its outside, passes without fear. There again, the road is dry.

Which means I should give the Boxster a second chance. Yes, today I can trust its adhesion and feel a clearer build-up of cornering forces, and yes, I can commit more completely to corners. But it still feels a little loose and clumsy, its mass threatening to take over. With the Porsche, you negotiate. With the Elise, it's an open dialogue.

The biggest problem with the TT course is that it's busy, being made up of main roads. So we head away from the motorbikes' tyre tracks in search of quieter spaces. Turn right straight after the tramway, be aware of the sheep, and you'll soon find yourself going down the old Tholt-y-Will hillclimb after a brief moorland fling. Here the Lotus's steering really proves its level of resolution; almost invisible ruts in the road tug at the wheel rim, some loose chippings signal their lack of grip even though there's no turning loading on this straight road. It's truly uncanny.

Then we drive the ridge road that starts at Brandywell, bumpy but very fast (again, sheep permitting). In the Boxster it's a creamy blur, extraneous inputs filtered out, efficiency incarnate. It's an effortless sort of pleasure. But in the Elise you feel every irregularity as you bond with your temporary road-race track, senses battered, exhilaration sky-high, reality registering but somehow reprocessed. On the long drive home the next day, roof still in the boot, I'll forgive the Elise everything.

Could a Boxster owner live with an Elise? I guess it just depends on the roads.