That old chest nut Squirrel, or even upright; [next you'll be saying which freezes fastest, a cup of hot water or a cup of cold water?] Depends entirely on how often the door is open. A full fridge (or freezer) has more thermal mass, and less air to rush out, so it's a good idea if the door is opened frequently. If the door is rarely opened ... packing it full might reduce internal convection, saving loss at the walls, but otherwise, not much difference...