Compare the BMW 6 Series M6 Convertible 2d SMG E64 - [2005] vs Alfa-Romeo Stelvio Quadrifoglio 2.9 V6 Bi Turbo - [2018]. 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 BMW 6 Series M6 Convertible 2d SMG E64 or the Alfa-Romeo Stelvio Quadrifoglio 2.9 V6 Bi Turbo. We'll leave it up to you to decide which is your favourite.