Like everyone says, it depends on the snow.

Comparing a competitive skier to a regular fat bike rider won't work (there are no established professional fat bike riders in the field you want to compare professional skiers to. It should also be noted that most competitive ski sports take place on groomed or packed snow). A competitive fat bike rider in the deep snow would likely be a relatively light rider. Race the fat bikes downhill in the spring, on a snow packed winter single track, or slopping around the slush and winter mess in the city and a much different rider would probably be much more successful.