I think the bike has some effect. I have two bikes set up for winter riding - an old Stumpjumper on which more of my weight is over the back wheel, and an old road bike, where I've got more weight up front. For sloppy, nasty days, like we've had here lately, the Stumpjumper is much more stable than the road bike for plowing through snow. The fatter tires at a low psi help with that, too. But now that I'm on my third winter riding year-round, I'm starting to see how much more of a role skill (as opposed to gear) plays in staying upright, specifically using momentum to get through tough spots and, in general, riding with confidence. I also find that spinning in a lower gear helps me get through the heavy stuff.