Used test – sublime soft-tops: BMW 6 Series vs Mercedes-Benz SL vs Porsche 911

Hefty depreciation means big convertibles often make great used buys. But should you choose the luxury of a Mercedes, the prowess of a Porsche, or the all-round talents of a BMW?...

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Alex Robbins
18 June 2016

Our Verdict

This is a difficult test to judge, simply because the Porsche’s cost – both to buy and to run – skews things so considerably. Indeed, we did wonder whether we should even include it in this test, so vast is the difference. But on practicality and usability, it more than holds its head up against the other two cars, and so exceptional are its performance and handling that it is worthy of consideration – even with its vastly inflated price.

That might mean you have to look at an older 911 for your money than you will a 6 Series Convertible or SL. Or, you’re in the lucky position in which money is no object, then you might be prepared to splash out on the Porsche where you wouldn’t on the other two. Were that the case, you wouldn’t be disappointed. It is an exceptional car, a cut above the Mercedes and the BMW in the way it drives and the way it goes, and a genuine sports car rather than simply a sporty drop-top.

For all that, though, we simply can’t judge it as a more savvy used buy than the other two cars here. It doesn’t better them by the order of magnitude that its price and running costs demand, and while we wouldn’t blame you if you wanted to spend the extra to get the most enthralling car here, neither could we recommend that you did so if all you wanted was a comfy, luxurious two-seater.

By contrast, the 6 Series Convertible is the value proposition here. Next to the 911, its running costs are almost city car-like, with impressively high fuel economy, low tax, and cheap servicing costs. It’s also the cheapest to buy. But cars like this need to be comfortable, and with a ride that never fully calms down and vibrations aplenty, that’s an area in which the BMW struggles to deliver. We can’t argue with the good financial sense the BMW offers, or indeed the fact it’s a truly classy cabrio. But it isn’t the best used buy here.

That honour must fall to the SL. Its metal roof makes it quiet and cosseting with the roof up, and with it down, the lack of buffeting is incredibly refreshing. Superb seats make it comfortable, too, and while it can’t hold a candle to the Porsche in terms of excitement, it’s still entertaining enough for most drivers. Indeed, only a slushy gearbox spoils things slightly, but given the nature of the car, even this flaw is one we can overlook.

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Also consider:

1st – Mercedes-Benz SL 350

Used test – sublime soft-tops: BMW 6 Series vs Mercedes-Benz SL vs Porsche 911

For Superb sense of isolation, roof up or down; impressive interior quality; great seats

Against Outperformed by rivals; gearbox can be found wanting

Verdict A brilliant long-distance cruiser, a lavish sun-longer and a surprisingly agile roadster – the Mercedes SL does it all

Rated 5 out of 5

Read the full Mercedes-Benz SL review

Find a used Mercedes-Benz SL for sale in the What Car? Classifieds

2nd – BMW 6 Series Convertible 640d

Used test – sublime soft-tops: BMW 6 Series vs Mercedes-Benz SL vs Porsche 911

For Lowest purchase price and running costs; spacious, classy interior; immense engine

Against Too much body flex; ride too firm; doesn’t feel special enough

Verdict Delivers in almost every respect, but the unsettled ride is hard to forgive, and it lacks the sparkle of the other two

Rated 4 out of 5

Read the full BMW 6 Series review

Find a used BMW 6 Series for sale in the What Car? Classifieds

3rd – Porsche 911 Cabriolet

Used test – sublime soft-tops: BMW 6 Series vs Mercedes-Benz SL vs Porsche 911

For Incredible performance; sensational handling; tactile controls; roof-down smoothness

Against Exceedingly costly to buy and run; narrow seats; cluttered switchgear; hopeless rear seats

Verdict A sensational piece of engineering and by far the best car here to drive – but even so, it’s hard to justify its stratospheric cost.

Rated 4 out of 5

Read the full Porsche 911 Cabriolet review

Find a used Porsche 911 Cabriolet for sale in the What Car? Classifieds

Specifications: Mercedes-Benz SL 350

Engine size 3.5-litre petrol

List price when new £72,495

Price today £28,000

Power 302bhp

Torque 273lb ft

0-60mph 6.3sec

Top speed 155mph

Fuel economy 37.2mpg (Official average)

CO2 emissions 176g/km

Specifications: BMW 6 Series Convertible 640d M Sport

Engine size 3.0-litre diesel

List price when new £72,630

Price today £26,000

Power 309bhp

Torque 465lb ft

0-60mph 5.2sec

Top speed 155mph

Fuel economy 49.6mpg (Official average)

CO2 emissions 149g/km

Specifications: Porsche 911 Cabriolet 3.4 Carrera

Engine size 3.4-litre petrol

List price when new £79,947

Price today £58,000

Power 345bhp

Torque 288lb ft

0-60mph 5.1sec

Top speed 178mph

Fuel economy 30.7mpg (Official average)

CO2 emissions 217g/km

Price today is based on a 2012 model with average mileage and full service history, correct at time of writing