are you going to mod either car? if so go with the S4. Its a Turbo.... you can get 400hp out of an S4 without a sweat. You have to drop some serious coin on the M3 to get those kind of numbers. I think the M3 looks a lot better. Out of the box the M3 handles better as well. But I guess if your doing mods a few suspension upgrades would tip the scale towards the S4. Both are great cars from great makers. I think the M3 has something the S4 cant boast and thats a race bred haritage. For me its all about the M.