Being a fellow Canadian, I understand the appeal of xDrive in our harsh winters. Now personally I find RWD and winter tire combo is good enough, given it's my personal setup right now. Facts are however, that a XDrive car will be better than RWD. How much better and whether or not that difference between the two is worth it is totally up to you. If you've grown accustom to your 135i's RWD dynamics in winter, a M2 should be no problem for you (it'll actually be better due to the added LSD). However if you need the AWD traction, the M2 will not be for you.
M2 pros:
-better driving dynamics
-better track performances
-will look more attractive (subjective)
-more fun
M235i XDrive pros:
-greater safety and capability in winter
-probably better fuel consumption
-lower cost to purchase as well as cost to maintain
-available right now (vs. a wait time for M2)