Compare the Mercedes GT AMG 63 S Turbo 4MATIC+ - [2018] vs Audi A6 RS6 Avant Performance 4.0 Turbo - [2015]. 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 Mercedes GT AMG 63 S Turbo 4MATIC+ or the Audi A6 RS6 Avant Performance 4.0 Turbo. We'll leave it up to you to decide which is your favourite.