M3 sedan would have more room. Good all around car.Coupe, just about the same room as a Mustang. But it will be older. (in your case) Just remember, even though your paying $40grand for it. Your still maintaining a $70,000 car. Oil change even if DIY oil and filter will cost you atleast $100. Parts and labor will be more expensive. It will be out of warranty soon. It will need repair that you need t pay for soon. If you plan on modding the car, It will cost you more.
Mustang, would probably be more fun on the street since your not tracking it. Would cost less to own. Brand new, that means its under warranty longer. Big downside is that everyone in their civic and camaro will probably rev and and want to race you. (might be a good thing depending on your preference.)
If your asking me, I prefer the M3, I bought the M3 because we needed a car for the family and I refuse to buy a boring car. Mine is a 08 sedan. So far no major issue I have 37,000 miles on it. I did not mod the car. Im keeping mine stock. It is the family vehicle. The wife drives it, I drive the suburban. I have 2 fun car on the side.
In the end I recommend you drive both. Buy the car for yourself and not because you care what the other driver or what the people walking on the sidewalk thinks.
BTW most test, shows both car being equal in performance. 1/4 mile times. Track times. But the good ol ass test will say mustang is faster on straight, m3 on the turns.