My opinion....Cancun is only good for the airport. It is like Vegas meets Miami except more people speak English in Cancun. PDC has a much more "Mexico" feel to it while still having a lot of tourist comforts. The area by the ferry is fun for shopping and there are many restaurants/bars/etc. There are a ton of things to do near PDC. There are caves, ruins, snorkeling, ziplining, cenotes, awesome beaches, etc... If you're comfortable renting a car, these places are very easy to visit on your own. You can drive to Akumal and snorkel with big turtles very easily and for free. Ruins are <$10 each. You can also sign up for a tour to any of these places which is good but more expensive. I'm not sure when you're going but another amazing thing to do in the summer is to snorkel with whale sharks. This is the one thing that might be easier to do from Cancun but I still wouldn't stay there. The tour operators in PDC will take you to the whale sharks. If you have people in your family who aren't good snorkelers, you may want to ask some questions though. When we did it the water was rough.