Hey man. Here's my opinion. I have has the M3(Titan M3, OSS Designs Buikd) I frigging loved her(yes, her.). I sold it only because I got an offer I couldn't refuse and had a kid, wanted something a little more sophisticated. Bought a 550i and built it again through OSS(Search for the M3 and OSS 550i Build). BMW is my favorite car, and I AM coming back to it.. However. The RS5 is utterly sexy. Drives extremely well, and has a presence about it unmatched by any current model bimmer. A tenth of a second here or two seconds on the track really doesn't mean anything to me. Unless you are the Stig, get the RS5, it's newer. More exclusive as they are only releasing 1300 of them, and is a very fun car to drive with much better technological features.
I got one and do not regret it one bit. That being said.. When the new M3 comes out OR when the M6 becomes more negotiable, I fully intend on making my return to BMW. Whether I keep the RS5 or not, I will decide at that time.
Bottom line: get it.