For serious diving or even snorkeling I would go with the catamaran - I think the other is more about the sailing on pirate ship experience. You will see more from the Catamaran and I am guessing the focus is more on what's below.
Although I could be wrong - but even so, wouldn't you rather go fast on a cat than be with a bunch of kids playing pirate?
I am the editor, but I also speculate, ask questions and play devil's advocate. I reserve the right to change my mind.