This is easy
Having owned a 2007 335i, 2009 M3, and current owner of 2012 Cayman R and 2013 C63 I have your answer.
The Cayman will be tons of fun to drive-like strapping on a go cart. The power will not be as much as the M3. The Cayman is a daily driver for one. The M3 will feel like a boat after getting out of the Cayman. Luggage space is tight on the cayman so if you see trips in your future get the M3.