Cancun: gov't manufactured city, great beaches, total hotel zone along beach separated from central party downtown with old section for original inhabitants before it went "Waikiki, MX." Average scuba, good snorkeling, great but expensive restaurants, all inclusives are good value but why do you want to stay at the hotel? Have to bus or taxi to downtown clubs (bus is cheap).

Playa Del Carmen: 1-1.5 hours from Cancun airport. European in style and visitors, smaller places you can walk to, central boardwalk, plus those who really know not to go to Cancun. Close to Mayan ruins in Tulum and the cenotes (flooded underground caves). Very good scuba. Excellent restaurants & not as expensive on average as Cancun.

Cozumel: Island across from the Yucatan mainland. Can ferry to Playa. Awesome scuba, average beaches but awesome wild water viewing on the back side of the island, where locals go... rent a jeep for the day. Very good scuba. Excellent restaurants & not as expensive on average as Cancun. Wonderful hotels along the strip from downtown to the cruise ship dock, with good pool action. Cheap taxis to downtown plaza area. Plaza Las Glorias hotel is a keeper! A scuba destination slowly being discovered by cruise ship day tourists.

An analogy you will understand (I'm an ex-Cowtowner): Cancun is Banff today. Cozumel is Banff 20 years ago. Playa is Jasper today.

They are all fun in their way. It's up to you!

Also: Sun Country Airlines is an option to look into if you are building your own flights. They fly to Cancun and Cozumel from many U.S. cities, including Dallas. You may be able to get a cheaper total flight package combining AA or United out of Calgary, depending on how much research/juggling you want to do.