I would go for the RS5. I've actually been looking at them for the past 3-5 days. We like what we like, and I like the RS5 more - looks and interior. And please don't go there with the "M3 will MURDER that car" business. The professional drivers are saying that the cars are very similarly based on performance, indicating the strengths and weaknesses of each. Some say it will come down to the skill of the drivers. How many of us have spent $1,500 on a single driver lesson? How many of us are more accustomed to driving on difficult roads other than these straight line American highways? Well, my hand is raised (I have youtube videos posted) ... how about yours?
Anyway, I think for me, the M3 is OLD and PLAIN. The Audi is NEW and FRESH. So, it's kinda like the new chick that is hired at the job - she becomes the most intersting one ... if only for a moment.