I own a '11 GT 5.0 as well, and agree with the original poster that the car is too softly sprung from the factory. Both the Ford and BMW dealerships allowed me to keep an M3 DCT and a GT 5.0 overnight so I have done a significant amount of back-and-forth testing between the two vehicles.
When I decided to buy the Mustang I did a significant amount of research on suspension upgrades for the Mustang to make it more M3-like. I ended up spending about $1400 on Koni sport shocks/struts and Steeda Sport springs. These were professionally installed for about $400 (including alignment).
The difference is night and day - the car is *much* more planted and feels very much like an M3: firm but not harsh. Just as important, the car sits much lower and eliminates the hideous wheel gap that plagues Mustangs. This is the way the car should have come straight from the factory.
So ultimately, this is my advice: if you're gonna get the GT, you owe it to yourself to upgrade that suspension as well.
Why didn't I buy the M3? A lot of it had to do with the fact that I was sick of dealing with BMW service (I had an '07 335i), found the M3 was not the scalpel I was hoping it would be. To me, anything over $65k is Porsche money and I felt more comfortable with a Mustang at $38k than a BMW at $68k.
But I do agree with Dave, it's not as good-looking nor is the interior as nice as the BMW. Gas mileage, while better than the M3 I drove, is not as good as I hoped. With the 3.73 gears I'm seeing 18-19 mixed....and I think I drive like a grandmother.