I would recommend you stay in Playa del Carmen it really has very beautiful beaches. Donīt forget to visit Mamitaīs Beach it is very nice. The ocean water is great no rocks, white sand.
There are a lot of really good restaurants in 5th Avenue.
Either way, you will have lots of fun staying in Playa del Carmen.
Maybe you would like to check this hotel out:
http://www.tripadvisor.com/Hotel_Rev...Peninsula.html