You really should go to TripAdvisor the crowd there specializes in this type of advice.

My two cents: We had narrowed down our choice to Cabo and Cancun for an upcoming vacation. Cabo is a lot closer timewise, but Cancun just sounds like a lot more fun. For one thing you don't need a rental car to get around Cancun but you will need one if you stay outside Cabo as the taxis are comparatively more expensive. Also, it does not appear that there is much to do in Cabo besides fish, golf and sun, and the beach in Cancun is reputed to be much nicer. In the end it was a no brainder for us and as it turned out the flights to Cabo were not much cheaper and the hotels were actually more $.

That being said, for many of us Cabo is a lot closer and therefore makes more sense for a shorter trip. Also have not heard of anyone who had a bad time there, though be sure to check out the reviews in the Starwood forum if you decide to stay at the Westin or Sheraton as the reviews are highly critical.

Whatever you do I would avoid PVR was there some time ago and the town is dull and nasty, the beaches not much nicer. Lots of families as you suspected, there are much more exciting places for a group of singles.