If you enjoy driving buy the BMW. If you enjoy looking at the car buy the Mustang.
The BMW M3 will blow the doors off a Mustang GT in every possible situation except maybe straight line racing. Introduce a few turns and curves into the track and the weaknesses of that Mustang will be all too obvious.
BMW = Driving Excitement
Mustang = Fast Acceleration
actually what that dude said isn't necessarily true, ford has really stepped up their game in the handling department, they finally made those things handle nicely. But he is also right in saying the M3 will probably beat it in the twisties, the mustang will just probably keep up a little better then it used to. Now i am a BMW fan ( own a 1982 320i) and i love that thing to death. but i also own a 1967 mustang coupe and there is nothing like the looks i get in that thing. both cars will get you in trouble, no doubt because they are both really fun and really fast. depends on what application you will be using it for