I just returned from a week in Cozumel, and I highly recommend it. The diving was superb and the people were great.
Of course there are two questions:
1) Are you a US citizen?
2) What are you looking to do?
1) Politics aside, be wary of visiting a country where you can't get any help from your country. My friend visited Cuba last week, and he said the women, and the food and the music were great. Of course the abject poverty was depressing in places. I hate to be a wimp, but "what if" something goes wrong? No embassy, no easy way "home" in an emergency, etc. If you are an adventurer, traveling with adults only, then go for it. Taking kids to Cuba? Not a good idea in my opinion. (and you did ask)
2) If you want to dive and eat and dive and sleep and dive and shop a little, then go to Cozumel. Great diving, with many fabulous dive sites and magnificent creatures, but not a tremendous amount of other things. However, Playa Del Carmen is less than an hour away by ferry, and there are DO's that can hook you up for tours to Chichen Itza (sp?) and Tulum plus diving at the Cenotes.
If you want to do more "other stuff" than diving, then Playa may be the ticket for you.
Cozumel has some nightlife, but not a ton for teens, so the kids would be "stuck" with you, and that could be "Good News, Bad News". Definitely not the tourist mecca that Cancun is.
just MHO