Is there any way to multi-mode commute? Even with a road or cross bike, you'll be slower in the winter, so you need to factor that in. If I were you, I would ride the mountain bike a few times to see if winter riding is something I really want to do. Aside from the speed difference, there are all kinds of factors to consider such as the cold, darkness, and potentially hazardous conditions (snow and ice). Personally, I like commuting year round, but I know it's not for everyone. Before buying a new bike, try it with what you have first to see if it's something you'll enjoy. If you like it and find your mountain bike is too slow, then by all means, look at alternatives.