RS5
Beemers are everywhere. Seems like every 19 year old kid in Southern California owns one (or more accurately, leasing one).
Audi is more sophisticated IMHO.
You'll probably be in the shop a fair amount for both cars. but Beemers are notorious for breaking down and requiring shop duty.
I'm happy with my E63 AMG

(though I'm yearning for a 911 Turbo...). 500HP for a 4 door sedan FTW!