What are the differences? Advantages? Disadvantages?
A downcut spiral shears downward and prevents the face veneer from splintering and fraying. It will leave a very clean edge and will require very little cleanup, if any. It doesn't do as good on pocket bottoms, though. An upcut bit does better if you're trying to get a very clean cut there. I use both, depending on what I'm cutting.
I rarely work with plywood except for Baltic Birch for Longworth chucks and on those I use a compression bit to cut the full thickness in one pass. A compression bit cuts up for a portion of the bit and down for the rest of the bit so it leaves a clean bottom and top. I also use the spiral downcut bits on Walnut, Maple, Cherry, Purpleheart, etc.