Compare the Mercedes SL Class 73 AMG V12 R129 - [1999] vs Audi A4 RS4 Avant 2.9 V6 Turbo - [2017]. 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 SL Class 73 AMG V12 R129 or the Audi A4 RS4 Avant 2.9 V6 Turbo. We'll leave it up to you to decide which is your favourite.