Both beach clubs are very good and among the most famous in Cozumel. I would recommend to check their websites for the included activities and the additional activities available....Playa Mia may be a little bigger but also offers in more activities, games, etc...good luck! Both clubs are feasible for teenagers.
Hi Bitsyb, snorkeling from shore at the Reef Club might not be the most spectacular snorkeling experience you can have in Cozumel, many shallow reefs close to shore were badly damaged by hurricane Wilma in 2005. More recommendable is to take one of the snorkel boats who head to the deeper reefs and the marine park, which are close to the Reef Club, this would offer much better snorkeling..good luck!