I have been driving a 2012 Mustang 5.0 (basically the same car as a GT500 where ergonomics/interior space/trunk space is concerned) rental while on my vacation for just over 2 weeks now. Although I have to admit it is a pretty good car, it definitely can't touch the M3 as a DD when you have to put stuff in the trunk and people in the back seat.
I do like the GT500 and it has a lot of performance for the money however it has little room inside and a small trunk. Put image, build quality, and all of that crap aside........ If you can only have "ONE CAR" the M3 is hands down better than the GT500.
IMO the M3 is superior to most cars on the road if you can only have one car....... now if you can afford to/have space for more than one vehicle then it is no longer superior to as many of those cars any more.
2011 E90 M3 ZCP - Individual Moonstone/Individual Amarone Extended/Individual Piano Black With Inlay:LINK!!!
1994 Euro E36 M3 Sedan - Daytona Violet/Mulberry:LINK!!!