If you are focused on modding for more power, get the Mustang. If you want a subjectively more refined and balanced driving experience get an M3. In my experience, no matter how much power you have, you get used to the feeling it gives you. The overall refinement and balance of a car is what gives sustained driving enjoyment.
Audi S6 * Audi S3 * Porsche Cayman GTS
--Former BMW M3 owner