I've now owned both a M3 and a GTR.. and I'm back to a M3.
Frankly, the GTR is a great car but I always had much more fun in my M3 on the track. The m3 has lower limits which are easier to get to and therefore make me a better driver. I think I'm never going away from a M3 again - its the perfect balance of daily drivability, track fun, strong reliability (no blown trannies) and just enough power!
Another weird thing for me was.. the day I sold my M3 and got the GTR, I badly missed the M3 (though had tons of fun in the GTR). Recently, the day I sold my GTR.. I didn't miss it at all - not one bit. And I almost cried when I got in my new M3
... atleast for me, I think I'm a M3 guy inside and out...