Compare the Audi A3 RS3 2.5 Turbo - [2018] vs Mercedes CLS Class 55 AMG - [2006]. Find out which car is the fastest, most powerful, lightest or has the biggest engine in our top trumps style comparison page. Find out which car has the highest top speed or is the fastest accelerating, is it the Audi A3 RS3 2.5 Turbo or the Mercedes CLS Class 55 AMG. We'll leave it up to you to decide which is your favourite.