the only downsides to the GTR opposed to your M3 are
1. less usable space for rear passengers
2. looks will be subjective
3. jarring ride
4. I think more expensive maintenance
5. more expensive insurance
if you want all out performance and an easy car to mod to 10 sec 1/4 mile then GTR, I'm thinking of going that route also, I really like the 2012.