There is zero doubt the M3/4 is the better car.
I own an M4 today (I'm a coupe fan, period). I'm selling my M4 to buy the M2 not because it is a better car (it won't be) but because it is a different car and will provide a different experience. Different may be better or worse but it will be fun nonetheless.
To me, cars are all about the experience. I will track it extensively (like I do with my M4), keep it for 2 years and then get something else to experience... maybe another M4, maybe another M2, maybe something different all together. There are very few things more enjoyable than talking about, thinking about and driving the piss out of fast, fun cars