I'm a bit biased, and overly opinionated, but Truk rules. Can't speak to Yap...
The owner of the dive shop in my town has been there 14 of the last 15 years, this year being his 15th time, and I can see it becoming an annual pilgrimmage for me as well, as I'm scheduled for my second trip to Truk in May 2003, after going in May 2002.
We dove with Blue Lagoon dive shop, which is one of the 2 or 3 land based resorts. I'm not entirely convinced that a liveaboard is any better in this particular location, as it's not that large, and you're basically just limiting yourself to being on a boat when most of the wrecks are a short enough ride from the dock (15-30 mins). We had one day of exceptionally bad weather in the lagoon, 3-4 waves, and I was thankful I was going back to a dry hotel room on some solid ground. The group I was with also prefers deep diving to shallow, in which case you can't really do 4-5 dives a day, so one of the major attractions of a liveaboard is removed. That being said, I didn't venture off the grounds of the resort much, so not being on a boat didn't do me much good.
Yes, a lot of it can be deep diving, but there are shallower wrecks as well, and many of the wrecks start in very shallow water. Strangely enough, I spent most of my time doing macro, partially because the vis was bad the week I was there due to weather, and partly because I couldn't shoot that wide with the camera / strobe combo I had. There's so much coral growth on the ships that it's hard to find them at times, and as a result, there's plenty of small life around. The only thing I didn't see a lot of was pelagics like turtles and sharks.
Here's a link to my gallery from Truk. It's pretty limited, but it might give you an idea.
I'd highly recommend going at least once, it's a "must dive" IMHO.
Tom
On a side note, I'm just putting up galleries for Cozumel as well, so they might look like they're there, but they aren't as of 12:30 EST on 1/28/03.