And I think it really comes down to what you want. The GT500 is pretty nice by Mustang standards, but not on the same level as the M3. To be honest, I'm looking forward to getting out of the GT500 in the near future.
My biggest issue with the Mustang is lack of traction. The car makes crazy power throughout the rev range, but you can't put it to the ground. You really have to modulate the throttle carefully through both first and second gear to maintain traction. If you like melting tires the GT500 is very good at that.
There is another issue with the GT500 that appears to be fairly common and that is a grinding when shifting quickly from first to second. I have encounter this problem on more than one occasion. Once I was totally locked out of second until the revs got below 3000.
The GT500 is a nice pony car. i don't want to take anything away from that. It is just night and day different from the M3. I have discovered I'm more of an M3 guy than a muscle car guy. I don't regret owning the GT500, but I am working hard to get into something more my style like another Nissan GT-R or maybe even another M3.
Feel free to PM me if you have any questions about the 2011 GT500. Mine is torch red with almost all the options including the SVT Performance Package and Nav. The only thing I my car doesn't have is the glass roof.