I'd have to go with the Audi to avoid the punter-jackass stereotype that follows M3 ownership..... OH HAI ANDY!
In all honesty though, I do prefere the subtle styling of the Audi (all Audis). The New M3 has too much of a HEY LOOK AT ME factor. For the love of jeebus, it's got a bare carbon fiber roof:
http://www.pistonheads.com/pics/news/16457/roof-L.jpg
so i have to disagree, when comparing '08 to '08, they aren't similar in form or function. The M3 tends to be more of a brute, especially with that engine.
The S5 gas pedal puts you back in your seat with an evil grin and a sense of control. The M3 (with the gadgets turned off) doesn't have a gas pedal, it has an "AAAAAHHHH!!!!!" button. I like both, but I've alread got a car with an "AAAHHH!!!" button. My only gripe with it would be the torque, but it makes up for it by screaming to 8000+rpm.