If you want a relaxing day hanging out at a beautiful beach and having a few drinks and some good shopping, then I'd go to Mr. Sanchos. I was at Mr. Sanchos this past Valentine's day, food was good, drinks were reasonable, beach was very nice, and the shopping at the front was very good bargain hunting, the best I experienced in Cozumel.
BUT, if you want to snorkle and check out some of the other natural wonders of Cozumel, then go to Chankanaab. Mr. Sanchos is not good at all for snorkeling.
it all depends upon what you are looking for. Both are really nice beaches. Mr. Sancho's has more water toys, more like a beach party.
Chankannaub has better snorkeling, there is an aquarium, botanical gardens, replica of the ruins, sea lions, souvenior shop. good food
We visited both in Sept. 2004. We started the morning in Chankannab and it is a beautiful park, however in my opinion the snorkeling was boring. Very few fish not colorful at all. We then took a short taxi ride to Mr. Sanchos. It has lockers, showers, restrooms , a swing bar and a pool. We selected a table at the waters edge, it was perfect. The waiters are very attentive. the food was reasonable and very good. They also had water sports, jet skiing, banana boats, para sailing, ATV , horseback riding and more. We went parasailing and it was the cheapest then all the other islands $50 per person. I believe you went up 600 feet . We are going back in May with the kids and are heading right back to Mr. Sanchos. Check them out on the web, www.mrsanchos.com. I sent them an e-mail asking them about the ages for ATV tours for kids and they replied within the same day, with rates. I can't wait to go back!!
Nachi-Cocom is just past Mr. Sanchos. It's quieter and less crowded and has more trees for shade (and hammocks). It's close enough to walk to Mr. Sanchos. We found that the latter had more of a party atmosphere. Neither are good snorkeling.