My girlfriend and I live together
I have an 05 M3 ZCP and she has an 11 X5 M and to say its nice to have those two M vehicles in the garage is understatement. Yeah....she didn't need the M but now that she has it...she'd never go back to a regular BMW!
The X5 M is an absolute MONSTER! Every time I drive it still amazes me how fast it is and the way it handles like a sports car. Tons of grip and hides it weight fairly well for being an SUV. Its just badass!
As far as winter goes, she has another set of rims (same size as standard front but for the back too) with the Pirelli snow and ice tires mounted and they've been just fine in the winter here in Maryland.
Anyway, if you want and can afford to keep the M3 and get the X5 M...its a no brainer.
Good luck nonetheless!