Used Convertibles: Mercedes SL vs BMW 6 Series vs Porsche 911

Take advantage of big-scale depreciation to bag one of these big-ticket convertible

Like the idea of a deluxe drop-top at a temptingly dropped price? These three five-year-old German offerings promise driver excitement, comfort and luxury at a fraction of their new prices.

In this rarefied market, even entry-level cars never leave you wanting for performance. The 3.5-litre V6 petrol version of the Mercedes SL is relatively gentle off the line but above 3500rpm the acceleration is much more urgent. Apart from the pleasing metallic exhaust note, it’s not noisy either. The seven-speed automatic gearbox is quite slurry, but you can pick Sport mode and do your own shifting with the steering-wheel-mounted paddles. Try to push it too hard though and the transmission simply won't have it.

You wouldn’t believe that BMW’s twin-turbocharged 3.0-litre engine runs on diesel, it’s so smooth. It’s nearly as refined the Mercedes, actually, and much more responsive courtesy of the torque it pumps out from only 1500rpm. It runs quickly to the 5500rpm red line before the smooth-shifting eight-speed automatic gearbox takes another gear.

You can put the Porsche 911 into fifth gear at 25mph and hold your right foot to the floor all the way to a rev-limited 166mph. It’s a seriously strong and flexible engine that’s as happy to be revved out in the lower gears as it is to be left in the higher ones.

The SL’s cabin looks great with its neat gear selector, big air vents and swathes of soft leather on the very supportive seats. Drop the roof and raise the wind deflector and windows and no draughts will spoil your serenity, even on the motorway.

The 911’s centre console and grippy sports seats create a snug environment, but some drivers will experience backache. Accessing the aircon and radio controls is awkward when the gear stick is in any of the odd-numbered gears, but the fabric roof effectively blocks out noise, and top-down wind blast isn't an issue with the deflector in place.

Much of the 6 Series’ dash layout and switches will be familiar to owners of cheaper BMWs, which is fine as the clarity and quality are so good, irrespective of price. The iDrive rotary controller easily controls entertainment functions via the central monitor. Although the 6 Series Convertible is unique here in providing plenty of space for four and a small amount of luggage, the passengers will have to tolerate a fair amount of wind buffeting. The 911 and SL struggle to hold more than a couple of soft overnight bags.

On cost, the 911 is more than double that of the other cars, with the 6 Series Convertible just beating the SL to the title of cheapest of the three. The 911 holds onto its value much more tenaciously than the other two, which is a good thing in one respect, but you’ll still lose less cash on the Merc or the BMW overall because you won’t need to spend anywhere near as much to buy them.

The 911 is also the heaviest fuel user and has the highest tax of our trio. Servicing won’t be cheap either, though the lack of a fixed-price servicing option for the SL will probably make it even more expensive than the Porsche.

The 6 Series Convertible is the cheapest, with the smallest purchase price, the lowest fuel consumption and tax and the least punishing servicing costs. The SL falls in between the two, though it’s nearer to the Porsche than the BMW.

BMW 6 Series Convertible 640d M Sport

List price when new £72,630

Price now £26,000

Available from 2011-today

Mercedes-Benz SL 350

List price when new £72,495

Price now £28,000

Available from 2012-today

Porsche 911 Cabriolet 3.4 Carrera

List price when new £79,947

Price now £58,000

Available from 2011-today

‘Prices now’ based on average mileage 2012 models with full service history

Verdict

The Porsche is very expensive, and we did question including it in this test for that reason, but it got the nod on practicality and usability as well as on its remarkable performance and handling.

Nobody would ever be disappointed by splashing out on a 911. It’s a superb car, and a level above the other two in its driver experience, but at these prices it’s not an inspired used buy over the Mercedes or BMW.

The 6 Series is a great value proposition. Its running costs are real-world livable, but it loses out on comfort with a jiggly ride and a fair amount of vibration.

The Mercedes wins our comparison. Its metal roof works brilliantly up or down and the seats are terrific. It’s nowhere near as exciting as the Porsche, but most drivers will be happy with it.

1st – Mercedes-Benz SL 350 (five stars)

A wonderful long-distance cruiser or luxurious sun-worshipper, with very decent handling.

2^nd – BMW 6 Series Convertible 640d (four stars)

Works on most levels apart from ride composure or driving engagement.

3rd – Porsche 911 Cabriolet (four stars)

Fabulously engineered and easily the best to drive, but stratospherically expensive.

Tony Middlehurst is a writer for WhatCar.