I have ridden both my mtn bike and my road bike in the snow and I'd have to say that I feel much more comfortable on my mtn bike.
However, my road bike does not have knobby tires or studded tires or anything ... it just has plain ordinary road tires. It cuts through snow just fine, but I felt extremely insecure when it came to more slippery conditions.
My mtn bike has large knobby tires which I flatten from their usual 50 psi down to 30 psi for additional traction ..... exactly like they do with the busses (coaches) that take tourists out to Fraser Island in Australia - Frazer Island is entirely made of sand. That seems to work very well, and I feel fairly comfortable.
My mtn bike is heavy, but that's OK because in the snow I'm not going to be moving all that fast anyway ... and both the snow and the heavy mtn bike give me a good workout! Come spring, when I get back on my road bike again, I feel like I'm flying!!
