in a couple of years, but my wife tells me that we are going back this August for a weekend getaway. We're staying at Cancun Palace as it turns out. I haven't stayed at Gran Caribe, so I can't help you there. For diving in and around Cancun, I'd use Scuba Cancun and will this time unless someone changes my mind. Their a good op and they offer day trips to Coz. Your other option would be to arrange diving in Coz with a Coz operator and ride the ferry across on your own. There are alot of great operations in Coz and a quick search of these and other forums will yield boocudles of info on them.
And of course you always have Aquaworld in Cancun. They also offer day trips to Cozumel. But they have mixed reviews on safety and number of divers per boat. My wife and I did a Jungle Tour with them a few years ago and we had great time and I thought they did a pretty good job. Never dove with them though.
As far as taking your gear goes, that's up to you but I'd take mine. Why? Because I know it's history, know it's been serviced, works correctly, and I'm comfortable with it. But as a bare minimum, I'd definately take my mask, regulator, computer, and fins, in that order.