The one reason I'd keep an M3 over a GTR is because of the people who drive them. My car is 8 years old, and still to this day - I get nods and peace signs from fellow M3 drivers, regardless of whether or not it's the same model or year. I get the feeling you're a bit of a loner in the GTR, and won't have that sense of comradery.
Another reason would be the constant negative attention you get. One day, you're gonna find yourself on the side of a road because a fart can equipped Honda Civic thought he could out-maneuver you, and instead he just smashes your front end.