Welcome to the M club (btw you are already there, you just don't know it yet).
When considering what to choose between the 911 and the M3 (especially the sedan), you are looking at two different animals. If you are are comparing the two, you are wrong. When it comes to performance (handling, agility, braking) 911 wins hands down. If you consider practical aplications in real world, M3 demolishes the 911. The interior room, lugage, engine flexibility, drivability, and maintainance price ... the M wins.
Again its what you are looking for. There is no right or wrong answer. I'll stick to my M any day of any month.