My parents have owned all sorts of bmw's. 3's and 5's. Right now they have a Dinan tuned six speed 540 with 325 rwhp.
Here are reasons for the m3:
1. There is no comparison in the interior quality of the m3 and the mustang.
2. You'll score more looks in the m3.
3. Better handeling.
4. I would have to go for the m3 for quickness. the mustang is quick, but the m3 has insane top end. by the way, they wiegh around 3500 lbs and rate the hp at the wheels (something around 230?).
Anyway, when you ride the m3 around you'll really feel like you are driving something special. I can't say the same for a mustang GT.