Thanks to you all! Let's suppose I do both, what's the most optimal way to arrange the visit? Here are the parameters. I'm planning to stay there for about 11 nights, say 5-6 nights on each side. I'd like to dive almost every day, maybe sometimes twice. Do multiple Cozumel wall dives, some night dives, Cenotes at least once or twice, but also visit Chitzen Itza, whatever other non-marine attractions that region has to offer, experience some local culture, some night life. Given my interest in diving, I'd probably want to spend most of my mornings in Coz.
One option seems to be to fly to Cancun (non-stop flight), get some kind of local transportation to Cozumel, stay there for 5-6 days, get the ferry back to Playa, maybe then rent a car to do sightseeing, and spend the remaining days diving Cenotes and/or taking morning ferry to Cancun, to do more wall diving. Does it work? Also, how much pain will it be with one big and one small person dragging 4 heavy suitcases + 2 carry-ons? Is there some way to get all the way end-to-end from the Cancun airport to Coz hotels via van, or some such, to minimize inconvenience?
Another option, I don't know if better, would be to rent a car in Cancun, and travel over the car ferry. If we're spending 5-6 days on Cancun, this seems reasonable. Or not? Is the car ferry still running? I think I've seem some post claiming it's not.
Yet another option would be to fly to Coz, then fly back out of Cancun... flight to Coz take a few hours longer from where I live, not sure if this is really better than taking the ferry.
Other options? What would you suggest? Does the possibility of el Nortes somehow factor into the answer? How many days of el Nortes should we expect at the end of December during this visit? Do the Coz operators offer diving Cenotes as a backup if the port is closed? Or if not, is there enough time in the morning to take the ferry and hook up with an operator on the other side?
The rest of Playa time is Beach at Tulum a couple days, shopping and some nightlife 
