It will depend on what you want to see, we did Yap & Palau Oct-Nov.
Yap I found the currents a lot stronger in the channels (Manta watching) than I did anywhere else in Palau other than Blue corner. Out of six dives to see Mantas we saw them twice. Sharks yes if you do a shark feed, a few large fish and small amounts of coral. If you like macro there is one site called macro 1-2, nudis everywhere and they are big. Mandarin fish you would be assured of seeing them. Dive op Werner Lau. excellent service four to a boat. The only down side, management is German and all guests other than my wife and I were German. We never got asked where we wanted to go, but there was a lot of talking in German. 4 star
Palau. the down side to Palau is the long runs out to the dive sites about 45 min, but even then its not that bad the route is very scenic going through the rock Islands.
Blue corner with all the hype on the board I was expecting it to be a lot worse than it is. Marine life great coral great. The dive op we used Neco Marine all I can say is 5 star all the way.