one of my bros has a 2004 m3 as well as one of my friends...its an awesome car if u need more room but still want a sporty car. In my opinion the coupe is much better looking then the convertible though
Also they're really safe and easy to drive in the winters. Both cars are equally fun to drive even though i still prefer the boxster, but if you ask my brother or friend they'll say they would never drive a box over their m3's. it all depends on what you want...i say go test drive one, they have a lot of potential for mods and overall if you need more space i think you'll like it.
One thing to remember is its a totally diff type of beast when comparing it to the boxster, when i drive my brothers i feel like im in an suv, to me it seems much higher from the ground (my box is lowered though) and of course its quieter then the box. I like the fact that i can actually feel the motor when i rev my engine, in the m3 its not really that kinda feeling.overall it performs really well and I do have to say is more comfrtable to sit in then our cars haha
Also, its a little cheaper to mod and maintain so i guess that could be a plus
Good luck with your decision