As an old sage used to say..."Girlfriends come and go. But, a car has to last beyond a couple of car payments".
Enjoy your youth! Get the M3, sedan or coupe, it doesn't matter which but don't be seen in an old man's car like mine (F10).
FORGET about any M3 being a "multipurpose" car. You just ain't going to want to use it for any girl friend things, pets, shopping (oh yeah, she'll shop), tailgate parties, or even pick-up games down by the park. Use your girlfreind's beater Honda for that or, as 'numbers guy' suggested, pick up a cheap 10 year old Taurus kind of car for that stuff. And, unless your girlfriend is Dana Kilpatrick, she just won't get it why the M3 is not "just" a car. Yeah!