Alot depends on what you really want out of the diving. If you love wrecks, go to Truk for at least a week. If you are more interested in seeing mantas or sharks or other creatures, go to Yap or Palau. I would probably tend to avoid the mainland sites and places like PNG where the politics are less stable at least for the foreseeable future. Just based on what I have read, and knowing that I am more of a creature person than a wreck person (hate confined area diving) I would do a week on a liveaboard in Palau, and a land based week in Yap. Good luck with wherever you choose and have a blast!
AggieDiver