I own both a 2011 E92 M3 DCT and a 2014 RS5. My personal thoughts on the RS5 in comparison to the M3...
RS5 pros - amazing exhaust note, more confidence out of turns and quicker off the line (AWD), more comfortable ride, better interior/toys
RS5 cons - less responsive throttle, less nimble (heavier), no frozen black paint
Both are great cars, both bring a smile to my face, but which is better? That's like asking if I like blondes or brunettes (the answer is all of the above).
Drive both and decide for yourself, it doesn't matter what some dude on the internet says.
BTW, best I can tell, most non-enthusiasts can't tell the difference between S5/RS5 and 335i/M3. Even though it's obvious to the rest of us.
Burn rubber, not your soul