I went the other way..from 997.2S to M3
Could not be happier!!
It is true that you should experience a 911. It is a cool car and very different than the M3. Not many things are really "different" nowadays!
But I dare say that the 997 is inferior to the M3. Just my opinion, so don't shoot me. I made a list of all the good/bad from street to track, and all I got was that the brakes on the 911 are awesome and better than the M3......
In most ways, the Cayman might be a better way to go, since the chassis is so sweet!
I gotta say, The 991S is an awesome car. (surely my next one!!!)