Most, if not all, of your sand filters need a vertical (top) discharge, and most, if not all, of your cartridge filters need the horizontal (side) discharge. I'm not sure about DE filters, but I would think they need a vertical (top) discharge as well.
EDIT: Jason is correct; I got tunnel vision and was thinking of the pad packs (pump and filter on a plastic base), because it's what I have. To keep both the pump and filter on the plastic base, I have to have a horizontal discharge pump, because I have approximately 6 inches between the two. If your pump and filter separated, then you could use either.