Haven't been to either so I'm only answering from my research. Paradise Beach has the water tramp and no entry fee. Seems like everybody loves it there .......great family beach. We looked into them all and chose Paradise Beach, will be there in less than two weeks. Again, haven't tried either, but research seems to show everyone loves Paradise Beach.
We were in Cozumel last January and decided on Paradise beach. I will say that there where alot of jelly fish there though. But then again I heard that from people who went to several diffrent beachs... I think it was the time of year. Just make sure you get some jellyfsh repellent. They sell it on the ship.
Paradise beach was nice. You could snorkel (not much to see) Climb the ice berg (it was hard) or jump on the trampoline. The food and drinks were not too pricy either.
I believe it was Playa del Sol that we visited on New Years and we enjoyed it. We took a cab from the pier and back. Our 2 yr old loved it! There was a small zoo (really small but we didnt bother), some vendors, a playground, a salt water pool, volleyball court, changing/shower facilities, bars, buffet, snack bars (the waiters even came to us) and several roped off areas on the water (each one had something). There was a snorkel area where they had "ruins" underwater to look at (we missed this), lots of water toys to rent (parasailing, jet skiis, etc), and there was an iceberg to climb as well as a trampoline (my son loved the trampoline). You can rent lockers, towels, umbrellas, and plastic molded not so comfy chairs (plus theres an admission charge depending on what all you want included). During the day they had several competitions for different ages (volleyball, games) and prizes were awarded (free drinks, massages, etc). Watch out for the chilli flavored lollipops and candy (ick) that they give the kids as prizes-I recently saw a recall on them because of lead! Really! The water was chilly, stirred up, and wavey, but it was Jan. Id go back again for my son's sake, but not for mine.