Mercedes-Benz has only itself to blame if this road test ends in anything less than unqualified praise. Had I driven the new and broadly excellent SL 350 on its own I might have mentioned its big sister, the £67,790 SL 500, only in passing. When I did it would, in all likelihood, have been to refer to the yawning £13,000 price difference rather than any notable deficiencies that might account for this gulf between the two.

After all, they are visually indistinguishable save for a tiny badge on the back that can be easily removed. Even their top speeds are the same, electronically limited to 155mph. So, same looks, same speed, save 13 grand — if only all life’s decisions were so easy.

Except it’s not that simple. Mercedes’ mistake was letting me drive the SL 350 before and after driving an SL 500. Those looks are mighty deceptive.

The SL 350 is, if such a thing can exist, the poor man’s SL. Its 3.7 litre V6 engine is identical in all respects to the 5 litre V8 in the 500 save that a couple of cylinders have been lopped off. Even in this emasculated form, it produces 245bhp which, as we know, is still enough to push the sleek roadster to 155mph. It will also scrabble to 62mph in 7.2sec. But while an electronic limiter means the SL 500 is ultimately no faster flat out it takes a lot less time to get there, reaching 62mph from rest in 5.9sec thanks to its 310bhp motor.

Oddly, if you drive very hard, the V6 car actually feels very little slower than the V8. It might have less power but it is lighter — the reduction is all under the bonnet conferring better weight distribution and more agile handling. Drive the SL 350 as fast as you can and it will be an uncommonly good SL 500 driver who gets away from you.

But these are entirely academic arguments because, the 500bhp £91,000 AMG version aside, serious drivers don’t buy SLs — why would you when you can have a Porsche 911 Cabriolet instead? No, for 30 years SLs have been about how they behave when driven slowly. To enjoy one to the full you must immerse yourself in the experience: fold the roof, lower the windows, extend an elbow and cruise. And this is where the SL 350 suddenly stops appearing to be quite so clever.

You notice the engine first. In the V8 the merest tickle of the toe produces a mellifluous rumble and instant thrust; do the same in the SL 350 and nothing much happens.

Press harder and it will accelerate but with a muted, nondescript noise. It requires more effort for less reward which is very un-SL. And should you wish to make an impression on your passenger, the SL 500 will explode out of the blocks, the 350 does not.

Then you start to notice where else savings have been made. The SL 500 rides, literally, on air, thanks to its having computer-controlled bellows where most cars have steel springs. You can, of course, specify air suspension for your SL 350 for £2,450 and having driven the car on both, I’d say it was more than worth it as the handling and, in particular, the ride are much improved.

The SL 350 also comes with a manual gearbox as standard, albeit one with electronic actuation. Automatic transmission, which is much more in keeping with the car’s character, costs another £670. But even if you do pay for the V6 SL to change gear itself, it’s nothing like as slick as the 500, requiring a needlessly heavy push on the accelerator before it will kick down and serve up meaningful performance.

Of course, all this should be seen in the context of how good the car is as a concept. Even in compromised SL 350 form it is the best convertible available at its price. Its looks, folding steel roof and engineering integrity add up to a compelling proposition and it has a better claim than any other to comprise two cars in one. Viewed either as one of the best-looking roadsters on the market or as an entirely convincing coupé, it makes sense on all levels, from showing off on sunny afternoons to everyday transport.

So, if the SL 500 is beyond your pocket buy the SL 350, tick the box on the order form marked “badge delete” and be happy nobody will ever know what lies under the bonnet. If, however, you’re wondering whether to buy the V8 or save £13,000, be assured that the SL 500 is not only the better car, it is also worth every extra pound and a few thousand more besides.

The SL experience has never been purely about how the car looks: if you want the complete picture you also have to consider how it goes and how it sounds. The SL 500 is far superior in both respects.

VITAL STATISTICS
Model Mercedes-Benz SL 350
Engine type V6, 3724cc
Power/Torque 245bhp @ 5700rpm/258lb ft @ 3000rpm
Transmission Six-speed manual with electronic actuation
Suspension (front) four links, coil springs, anti-roll bar; (rear) multi-link, coil springs, anti-roll bar
Tyres 225/45 WR17
Fuel/CO2 24.1mpg (combined)/281g/km
Top speed 155mph
Acceleration 0 to 62mph: 7.2sec
Price £56,340
Verdict An SL to look at but not quite to drive