We happened to do 6 dive days Palau (on the Aggressor) and 4 dive days Yap. I enjoyed Yap a lot and it was nice to mix it up a bit, especially when it's generally an easy add-on as far as flights. (Although the flight times suck and the Yap airport is awful especially in the middle of the night, not unsafe just a lousy place to be.)
It's always cool to see mantas, but I think it's a shame there is so much focus on them in Yap. We did see some mantas though the viz was pretty bad at the time. I wouldn't have been happy going there just to see that, but I've been fortunate to have much better manta encounters other places. YMMV. The mandarin fish dive was interesting, although we actually got a better look at them snorkeling during the day while on a kayak trip in Palau. But we did some dives on the outer walls of Yap which were great and I actually liked better than a lot of the dives we did in Palau. There were even lots of interesting critters to be found on patchy reefs near some of the manta sites. What I don't know is how often the dive ops normally do the other dives - I suspect not much if they've got boats full of customers that want to see mantas, maybe more often if people have seen their share already. I was with a group so we controlled where a couple of boats went. If I was going back there not with a group I'd want to get the answer to that, and it might determine my choice of dive op.
