Just test drove my buddy's 2012 GTR with a full exhaust, tune, and intake. I am absolutely speechless over that car, as where I once HATED the GTR (said it was a cheap nissan, no fun because of AWD, souless, etc...) I am in love with everything about it. Even though it is a bigger car, it is so nimble and does not feel its true weight at all - the car feels very planted and is very confidence inspiring and IMHO handles way better than my M3. The transmission is another plus, I personally think it is better than the DCT in my M3 as well. Maybe my DCT is shot from constant abuse and the power from the blower (extremely clunky at low rpm, and clunks on some downshifts and has delays), but the GTR transmission is a touch more responsive than the DCT in my car and is significantly less clunky. Another thing I loved was the torque - even though my M3 is faster from a roll, if I do not start at 5500rpm against the GTR from 45-50mph I will not pull on him. The GTR has torque everywhere, and honestly appeases my appetite for power way better than my M3 does (probably due to the M3 not having any power whatsoever under 5k rpm).
Only complaint is the noise in the cabin - you can hear everything outside, and the car does not feel as luxurious as the M3 on the interior (M3 is better insulated and smoother feeling on the interior, as where the GTR is more rough and noisy).
In the end, for someone looking for a great performance platform the M3 does not compare to the GTR. Its amazing how my views of a car I absolutely despised changed after 5 minutes of driving it. If I were you I'd consider a GTR, it is most definitely a step up from a blown M3 (also another thing to consider is reliability - a blown M3 is going to be nowhere as reliable as a lightly modded GTR). I am keeping my eye on the 2013 GTR....