RS5 unlike other Audis is limited production. It's going to stand out more than M3. M3 will reward you more when pushed to the limit but I have never seen ANY M3 driver push their cars to the limit on public street yet. (Guys, how many of you oversteers onto the freeway?) Unless you are a track junkie, go with the RS5 in my opinion. The engine sounds terrific, the style is best in class inside/outside and you get that exclusivity, at a price of course. Ultimately comes down to which car you like more, there's no subsitute for a M3 neither is there one for RS5.
Caveat is if you buy M3 now it's going to be outdated next year. I rather have the lighter and more powerful M3.
As far as reliability both are german so that goes out the door. Unreliable is part of the experience, JD powers just rated BMW X5 one of the worst reliable used car to stay away from. Audi has their own problems. I'd step up to a porsche if you want more reliable german cars.
- There's nothing in my pocket other than knives and lint