What is your intent with the car? Two very nice cars but still different enough. The big difference is going to be the AWD with the Audi. I've looked into Audi's a lot lately since I was wanting to pick up one myself. My fiance has a new A4 and it is a very well built car...I assume the S5 has the same quality. The Quattro is amazing in snow and with all the options it's a very comfortable car. If you're worried about the M3 outhandling the Audi (which I'm sure it no doubt would), you might look into Stasis Engineering. They have one of their shops down the road from me and I get to see them pretty often. I seen recently they've been driving around a black convertible S5 with their package on it...