If you live in a cold area of the world (snow, low temperatures), the RS5 is the better daily driver and the safest one of the two. If you want to play, then get the M3.
The M3 can be supercharged (ESS, G-Power) and do 300KM/h in less than 30s if you're interested. There are currently no known powerful tuning options for the RS5.
Out of the two I'd get the M3, I'm a BMW fan though. The RS5 is more expensive and as powerful as the M3. If money is involved then the M3 has gone one more plus.
Either car you'd chose you can't go wrong. Now it would really be a crazy idea to bring a C63 into this equation, wouldn't it?

 

RS5 gets my vote. I've driven them both. The M3 is good, but lacks torque and has a bad image IMO. The way I can tell the winner easily? I'd consider trading in my 911 for the RS5, but I'd never trade it for the M3... (I'll probably keep the 911 either way).

I think the RS5 has a much more interesting design, interior, and sounds amazing. I'd give the handling edge to the M3, but since I wouldn't track either, the RS5 makes more sense.

 

Very good points made above.
I'd only like to say one thing: get the one you feel that is the right car for you. When you have to live with something it's best you want to in the first place.

 

audi

 

Huge M guy (own an m5) but RS5 is just sexy.....

 

M3>RS5

Sorry not a fan of AWD sports cars.

 

RS5 without any hesitation what so ever

 

996/7 Turbo??

 

 

an option if you can deal with the interior. M3 w/ 6 speed out of those choices but if you're smart you could pick up a mint 996TT instead.

i would take a B7 RS4 over the RS5.