If replacing the M3 definitely do the E90. It is much more 'practical' than many people give it credit, especially with the fold down rear seats. I've been able to get a set of golf clubs, two suitcases, two computer bags, misc other bags and two 75lb dogs in it comfortably.
Best of ALL worlds in my opinion with giving up VERY little if anything to the coupe.
I had to look around for a while but managed to find an incredibly well equipped one (only missing sunroof and rear shades I believe) and couldn't be happier! Also be willing to "travel" to either ship or drive a car back. Opens up a TON of potential cars!