R8 4.2 and RS4 have the same engine. As for the RS4 v M3 debate, I have chosen the RS4. This is probably a letdown to an M3 forum, but I need the AWD and there are a lot more available in my area.
I know that the RS4 doesn't have as many mods, but with so few in the US, it is unique enough on its own. I heard a Tubi Rumore and it was simply amazing. I am planning on Tubi Rumore, ECS H-pipe, and Fast Intentions down pipes. Also the GruppeM air filter and a JHM shift shifter and tune. These, a carbon steering wheel and ADV.10's and I'll call it complete. Thanks for the opinions guys.