Cozumel... Hands down is better diving, there is many great dive companies and lots to see, all drift diving, shallow or deep turtles, sharks and tons of great things to see. In Mexico I would only dive in Cozumel. Cool trip from Playa del Carmen on the high speed ferry, land in St. Miguel turn right off the docks, walk about 200 yards and there is a great little dive shop. Have a Blast !!! P.S. you should try cenote diving while in Mexico. (bring your wetsuit)