This is an old chestnut, in fact there is an identical thread on here even now, which I think best merged, if I can remember how to do it, as a mod.
However, for us and our type of sailing we prefer up, because I can reach the galley for a top-up from the helm, and everything is in view. Also I can secure stuff there with a quick dash from the helm if I see a wake situation coming.
The main issue from a practical standpoint are the loss of floor space to steps every time there is a level change, so floor area is lost in galley down, and also potential stateroom space, as has been pointed out.
The real plus I would think in galley down in for true blue water travel, and therefore cooking on the go, because the galley down is then nearer the roll centre, and less affected by the sea motion. By definition, you would normally be doing that in a larger boat anyway, so the space loss is less an issue.