So many variables to consider when tire inflation pressure is concerned...

Long story short, if you are on pavement and expect that your tires can reach it, then it is preferable to do so rather than float on top of the snow, which means running with higher pressures so your tires can push the snow away. If, on the other hand, the snow is too deep to cut through or you are not on pavement, then a lower pressure that allows you to stay on top of the snow is best. Around where I live, there is often mud underneath the snow(lots of springs pushing water up even when most stuff is frozen).

If you want to get into the physics of it, consider why studded tires work well on ice: The studs are small points on the contact patch that create very high pressures, much higher than the rubber that surrounds them. The higher pressures at those points allows the stud to 'dig' into the surface giving you better traction. If you've ever seen the tires that are used on rally cars for winter/ice races, you'd see that they are very skinny. The contact patch is smaller and therefore psi is higher. Of course, when the terrain changes from glare ice to fluffy powder, the high pressure idea doesn't work very well at all cuz' ya just sink in the snow.

Just something to keep in mind: when your tire is inflated to 20 psi and that tire is carring 1000lbs., your contact patch will be 50 square inches, regardless of the size of the tire. This is why bigger tires allow for lower pressures and therefore better flotation. The same principle works for load carrying ability; in general, if you want to carry more weight, you need either more pressure or a larger tire. Another option, of course, is to use more tires to handle the extra weight, which is why we see dually pickups.

enough rambling... hope this helps clear things up.
Happy wheeling.