I say go with the M3. Bimmers have a huge following of enthusiest behind them, especially M3s. The BMWCCA actually host quite a bit of fun events.
I tracked my 335 and now my M3 and have never had a problem with either of them at the track. The 4 year no cost warrenty is a big deal to me also. Also, the fact that when my car goes in for maintenance, BMW gives me a loaner BMW to drive. Who else does that?
Personally, I think the Caymen in the Prosche lineup is equivilient to the 1 series in the BMW lineup. Its for people who cant afford a 911 for the most part. Alot of car shows and websites are doing a direct comparisons of the Caymen R and the 1M. If you get a Caymen then you will have less practicality, less performance, and a higher cost. Yes you might be in the "P" club but you will be at the bottom of it whereas if you got the M3, you would be at the top of the BMW club. I really dont see anything that that the Caymen can offer you more then the Bimmer.