You won't get that kind of action in Palau. Plus, there is very little dive pressure in Yap, and it is just a totally relaxing dive vacation. If you decide to do Yap, only plan on two tanks a day.
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I have to agree with Dan. Do all 3 if you want to. I am not a wreck guy, so I would rather stay in Yap over Chuuk if I had to choose. I like Palau--it is physically beautiful topside, has the cool jellyfish, saltwater crocodiles, etc.
I also really like Yap and the Manta Ray. It is one of the most low stress, family oriented places I have stayed. It is the kind of place where you can make good friends with the staff and other divers (e.g. Deep Water Dan was on my boat a few years ago). I have a daughter who was just learning (and was to dive & the staff, especially Jan, took her under their wing and made her trip. We took her to Yap from the east coast because we knew that the relatively calm water, beautiful reefs, big animals, crystal clear water, etc. is perfect. I would actually suggest Yap as a warm up to Palau--knock some of the rust off before hitting the currents and larger more impersonal dive ops. (I really like Palau too--but it is like going from a small town staying with a family on their farm to the big city).
One clarification to what Dan said--most people only do 2 dives in Yap--and usually they are long long dives. This doesn't mean that they won't take you out 3 or 4 times if you want to go out. I have been out alone--just me on the boat--diving with Jan from Manta Ray--because I wanted more diving on their outer reefs. Another time I wanted to play with my camera, so they took me alone to a part of the lagoon to do some shallow macro dives--just me.
Also the last time we were there my wife, daughter and I went out fishing for our down day before flying. Spur of the moment request--no problem as usual--and a couple of big wahoo (ono). Just reel fast or the sharks will get them (as my daughter learned).
With that there are down sides--MRB and Yap try hard, but will never be luxury dive destinations. The island is just too real for that. IMHO this allows you to really know the local culture much more than you would say from staying at a more tourist oriented economy. While much more urbane, even PPR on Palau is not the 5 star destination they claim--when we were there the hot tub attendant decided to test the hot tub by lounging in it & the luggage "bell hop" couldn't find the room so decided to take a nap instead of delivering the bags). Bring anything you can't live without (e.g. women may like some hygiene products at certain times of the month--they may not be available). Chuuk is a bit poor and can be rough around the edges. I will go back as soon as I can--just expect quirks. If you don't like quirks, and this is going to be your only trip in your life before you start to fall apart--then you may want to consider the Maldives (e.g. the Four Seasons there) or perhaps some of the upper end places in French Polynesia.