We have been to Costa Maya 5 or 6 times and returned in March for our first visit post-Dean. Basically, it is unchanged. While the beach village of Majhual was swept off the map,, it has been rebuilt. The services are the same - friendly, cheap beer and food, cheap massages, etc. Only difference is that all the restaurant/bars have been moved off the beach and across the street so there is more beach area. Each bar its own little piece of beach and waiters who keep an eye on you and bring you beverages and food.
I like it much more than Grand Cayman, but I'm very glad to have experienced Grand Cayman several times. Previous poster is right about Cayman - expensive, elegant, beautiful, fabulous snorkelling, sting rays and all the opportunity to send your boss a postcard from Hell saying "having a wonderful time wish you were here"
Both have their own place and flavor in the cruising world.
Can't remember if the previous poster mentioned that Cayman is a tender port.