i've ridden both in snowy/icy northern vermont (commuting and fun-riding). I personally preferred the fixed gear for the immediate feedback that you get (it's a lot easier to tell when you're losing traction on a fixed gear bike than on a bike with a freewheel). That is... unless fixed wheels blow your mind?

More important though is what kind of tires you're riding. Get yourself a pair of good studded tires, they'll be worth their weight in gold if they prevent you one time from slipping with a car behind you.