they seem similar in so many ways, I found it hard to decide which one to go to. we chose xel-ha. the food was included and there was lots of selection, my super picky DH ate well. for some reason I thought there was a lazy river where you leasurely float on a tube. in actual fact, the current will carry you for a bit but you end up in this huge lake and you really have to swim for it to get to shore. the cool thing is that they take your shoes etc and lock it in a bag for you and it meets you at the end of the river.
I was pretty underwhelmed by the quality of the snorkelling, it was nothing like "a living aquarium". But oh well, maybe I went to the wrong spot. The current was quite strong too.
My sister when to excaret and really enjoyed the underground river, said it was gorgeous.
I don't really suppose that you can make a bad choice, regardless you'll have a great day I'm sure.