I believe RS5 will be more unique in the long run... I also think it's prettier than an M3. I liked my S5 when I had it, and had they had RS5 at the time when i was getting an M3, i would probably go with the RS5.
To each his own.
2013 Audi //RS5 Phantom Black Current
2009 e92 ///M3 Jerez Black<~~Sold
2009 Audi //S5 Phantom Black<~~Sold
2002 e46 ///M3 Carbon Black <~~Sold
2007 Audi //RS4 Dolphin Gray<~~Sold