I was discussing this with my friend after the choices were presented to me. We chose ghost due to the following reasons;
-skellys are usually low lev mobs, tho freeport area is abundunt with the bony suckers
-zombies are also usually low lev mobs too, and they nasty
-ghosts are usually higher lev mobs, haven't seen much of them either so I figure I'd see more later
-easier to make a ghost mob from an already existing model, heh. Therefore, you'd probably encounter more later on as they add new areas and such.