Compare the Porsche 911 GT2 RS - [2017] vs Mercedes SL Class 63 AMG Cabriolet - [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 Porsche 911 GT2 RS or the Mercedes SL Class 63 AMG Cabriolet. We'll leave it up to you to decide which is your favourite.