Governments can mandate snow tires, only because they can not force you to sell your existing car and buy/drive an AWD car.
Snow tires will always trump all season tires in inclement
braking conditions (wet/ice/snow) - for both AWD or RWD.
AWD/xDrive/Quattro with all seasons will "most times" assure better performance over RWD on snows in
acceleration off the line, cornering, fish tail corrections, lane changes, hilly terrain, inclined roads and steep driveways or just plain getting yourself loose when you're snowed in at the grocery store parking lot!
On paper (!!!!) snow tires will perform better (whether it is AWD or RWD), but real life experiences suggest otherwise. Once your snow tires start loosing tread, you're SOL baby! AWD and all seasons are more forgiving under real life conditions (not only controlled, lab tested conditions).
My very subjective personal experience driving in New England weather, and the very well respected and widely published ///M235i snow-driving and universal rating-scale system says (citation required):
e46 330xi all seasons - 8/10
e90 330i all seasons - 4/10 (but boy was it fun!)
e90 330i dedicated snow setup - 6/10
f30 335i xDrive all seasons - 8/10
e92 M3 Michelin super sports - ... stay tuned. I hear the rear differential helps make loss-of-traction-power-slides-into-the-ditch sound much better, even faster and more glorious!
Moral of my post, do what you please! None of us will be there to defend your decision of tires or drive train when you're stuck in a snowy ditch on the side of the road in sub freezing temperatures and bad traffic late at night when (oops) you forgot to bring your gloves and winter coat along and you have to dig your car out of the snow bank or explain to the guy you just rear ended why your car swerved at him like a maniac ... its all you my friend. But i'm sure you will probably think of us too and some of the stuff being said today.
The onslaught of RWD vs xDrive threads is inevitable and I'm gonna go out today and stock up on some hot coco for the fun! Can't miss it!