Choosing between these flagship diesel coupes comes down to your priorities. With its smaller-capacity four-cylinder engine, the Mercedes is cleaner and cheaper than the Audi, so is a more affordable company car choice. And private buyers who go for the C-Class will be able to pocket the £4,835 difference in price over the top-spec A5, or indulge in Mercedes’ comprehensive options list.
Yet while both cars’ diesel engines produce 500Nm of torque, the 3.0-litre V6 in the Audi is smoother and more refined – and that helps the A5 feel more upmarket. In-gear performance is similar, but the Audi is punchier across the rev range and the dual-clutch S tronic transmission is a better choice for keen drivers.
The Audi has a smarter interior, plus it’s the more engaging driver’s car, with all-wheel drive, excellent body control and weighty steering. Unfortunately, the large wheels of our S line model result in an unforgiving ride – although the C-Class Coupe doesn’t ride perfectly, either, despite its softer suspension and smaller alloy wheels.
So the A5 clinches victory with its sharper looks and smoother engine, plus the genuinely upmarket feel it provides. But the Mercedes runs it close and still represents excellent value for money.
Audi A5 3.0 TDI ★★★★
The A5 has always mixed style and substance, and Audi has refined both, so it’s more desirable than ever. It looks good in S line trim, but you’ll get a better ride – and reduce the price premium over the Mercedes – by opting for an SE with smaller wheels. A 2.0 TDI or front-driven, lower-output Multitronic 3.0 TDI A5 will be more affordable, yet our test car justifies its higher price with its performance, refinement and quality.
Mercedes C250 CDI Coupe★★★★
Mercedes doesn’t offer its 350 CDI engine in the C-Class Coupe, but the C250 is a desirable car that runs the bigger-capacity A5 close for performance. It’s good value, too, yet trails on refinement, throttle response and handling. The solid cabin isn’t as classy as the Audi’s, either, while the ride can get fidgety on rough roads.