If I was living in Belgium I would definitely have chosen the RWD. If I were living in the Alps, or in Norway (which I am), I would choose the xDrive. The balance in the car is so good that on wet weather it's just more fun with RWD. In snow, it's also very fun with RWD, but I would have had the xDrive to add more traction, and make it a more usable daily driver.
My tip: Save the extra money the xdrive costs and spend it on extras to the RWD M135i
