A lot of BS bickering over stupid crap in here. I have a M3 and sell Fords so I have access to compare the two on a daily basis. As all the car mag tests have shown, the are VERY similar in performance. The mustang is a very nice car and super inexpensive. Ride quality lacks a bit but interior is best of the American made cars. It doesn't have that "cheesy" plastic feel to it anymore.
I personally don't like the interior layout compared to the BMW though and I also needed 4 doors. I would love the 6mpg bump in gas mileage though.
It all comes down to personal preference.