I would suggest traveling down to Playa Palancar and then negotiating a boat out to Palancar reef. While it is a world-renowned dive spot, it also offers great snorkeling, but you do need to get there in a boat.
Nachi-Cocum is much more laid back and quiet than Mr. Sancho's and Nachi-Cocum is now charging a fee for the use of some of their facilities. I guess it depends on what kind of experience you are looking for. The beach and environment at Nachi-Cocum is more appealing, but there were more options, activities and a livelier crowd at Mr. Sancho's.