I've been to St. Lucia and Playa del Carmen.

St. Lucia was okay.

Playa del Carmen was awesome. Beautiful beaches, laid back, fun little town, close to some fun touristy things to do, short ferry ride to Cozumel. The only thing is, you'll want to check out the beaches there; they had a hurricane a coupla years ago that really hit some areas hard and I'm not sure how they've recovered.