Having been to both Chank and Xel-ha, I can tell you the snorkelling is SUPERIOR at Xel-Ha, but do take into consideration the fact that you can do Chank for very small dollars and WAY less time and transportation. Having said that, I have done Chank the last two cruises, this year we're going to try Xel-Ha on our own, the Ferry schedule and a lot of faith are all I think it will take! You'd enjoy Chank a great deal if you'd never been there, it is truly lovely and a great way to spend a day. The reef is growing more and more barren as the human impact is more greatly felt over this last five years....and the other two sites rely a bit more on human intervention, whereas Chank is just a lovely national park that they certainly keep up with, but don't do a whole lot to enhance. It's probably more reflective of nature in the Yucatan Peninsula, but it depends really on what type of experience you want most. Enjoy!