Cancun will probably be more fun if you are into getting heavily intoxicated and meeting members of the opposite sex. Not surprisingly, I had a good time there.

But that may not be for everyone, you'll have to decide for yourself. It's a party town.
Playa del Carmen is about an hour south and is one of my favorite places, very laid-back and cool. Maybe worth the small cost of a cab ride to check it out for a day. There's scuba diving and other things to do as well...the Mayan Riviera is a great place.
Haven't been to the Bahamas, but have heard other reviews of them from friends/family. They are nice islands, but less "party" oriented, more "vacation" oriented, if that makes any sense. If it were up to me, I would pick Cancun and start getting inured to the taste of tequila.