Having been to both places, and an avid snorkeller/diver, perhaps I can help. Are you looking for color or quantity?
If you are "swim with the fish" people and love dazzling color, your better choice would be the Mayan Riviera -- particularly Cozumel although Playa del Carman and the Riviera are close enough. Cozumel itself tends to be very expensive (island) and you can find better/less expensive accommodations in Playa del Carman and on the Riviera and still have a day trip over to Cozumel.
Being on the caribbean side of the water, there is a much larger variety of sea life and the colors are more vivid and fluroescent. Spending all day in the water and never getting bored is easy to do there. And there are many places within a short distance to go so you aren't isolated. Beaches are idyllic. Party life on the Riviera (and Cancun is not too far away either) is legendary.
While Costa Rica is beautiful, the main touristy centres are located on the Pacific Side where there is an abundance of sealife although the colors are primarily browns, greys, black, white with the odd stripe thrown in. Beaches are black sand (not as bad as it sounds) and party life we found to be a little more low key.
It is not that I wouldn't recommend Costa Rica -- far from it. You should just be prepared for what you are going to see. Check your jellyfish schedule for the water. Sometimes there are plenty and you might want to consider packing a 1/4" neoprene bodysuit to help with the stings.
Perhaps a visit to your local diveshop will help you decide. Most have great resources on destination dives/snorkelling and the pictures may make it easier.