I test drove the Mustang (the GT 5.0 and the GT500...couldn't get my hands on Boss) when I was shopping for a car last year. I really thought I wanted a new mustang, however after driving the stangs, Cayman s, and M3 back to back I found that the bimmer had more of what I wanted than the other 3 cars. The e9x M3 really is a German muscle car with more polish and refinement than what is in the mustang.
By no means am I knocking the mustang. I truly think it's a great car. However the M3 is still a step up in all the areas that were meaningful to me.
Don't hesitate to buy a quality used M3. There are some great cars for sale out there at substantial discounts to their original 'new' selling price.