M3 is much more exciting and comfortable inside..
The GTR is extremely fast and handles very well but no thrill... i get the feeling like no matter what i do the car is gonna save me.. same applies to traction off.
If you love driving go for the M3
If you want to be the fastest and the best and let the car drive you go for the GTR
Had both prefer the M3, Heck.. my 335i makes me smile more then the GTR..
now: 2008 335i (E92), 2011 M3 (E92), X3 35i (F25),2012 E63 AMG (W212)
gone:2009 GTR, 2007 335i (E93) 2004 M3 (HPF stg 3), 2008 Civic Lx, 2007 Z4M Roadster, 2001 Gembala GT550, 2007 CLS 55, 2007 STI, 2004 M3