I really prefer using my hybrid in the winter over a mountain bike, but that's because the roads are typically cleared pretty quickly in Toronto (this winter from hell notwithstanding). Mountain bikes often come with front suspensions (completely useless for commuting and just add extra weight) and big knobby tires (take 10x the amount of effort to get where you're going compared to normal bike tires). Disk brakes are really nice to have in the winter though, as rim brakes just don't work that well in ice, snow, and slush. Make sure the bike you end up with has mounting points for a rack!