I think this is a great comparison. Lots of things to weigh here. Straight line performance would likely be very similar if not an advantage to the S/C M3 depending on your kit.
S/C M3 advantages:
Plenty of power
RWD (I see this as an advantage)
Interior (style, comfort)
AWD (for all-weather grip and track)
Quicker on a track
Race oriented interior, seats, etc.
For me the big thing is warranty. I know most of you don't care. But the S65 is pretty expensive. Would not supercharge my M3 for that reason - I'd go with the GTR. That said I am in no way looking for more torque, so I'm pretty happy. But if you're on a torque hunt and can afford a GTR I think you should go for it.