All I can say is that they are completely different cars, and Mustangs particularly have a very different driving sensation than most other cars.
Where the BMW is all about dynamics, technology, modern suspension, luxury. The Mustang is crude, rough, FUN, not every clown can drive a Mustang fast, almost any clown can drive a BMW fast and the car forgives the mistakes.
They are totally different animals, I wonīt say one is better (define better for "what"). Better engineered, the BMW, better fun to drive, personal experience, the Mustang definitely.
Go and drive one, then decide what to do.
2015 50 Anniversary Package GT
2014 Race Red Premium GT Convertible
2003 Silver GT
1981 Ford Mustang Fastback
1966 Mustang Coupe 289 4V
15 years ago 1984 Ford Mustang Hardtop