We have been to Costa Maya a couple of times...but that was BEFORE Hurricane Dean paid a visit. We have been to Grand Cayman about 15 times.
Both of these ports are safe.
But there is one huge difference....Grand Cayman is commercial and expensive. Costa Maya is still developing and less expensive.
Grand Cayman has the Margaritaville, Senor Frogs...beautiful 7 Miles beach and the prices that go with all of that.
Costa Maya "downtown" area that you take a short bus ride to has beach road with "some buildings on one side and the beach on the other."
Grand Cayman is where you swim with the sting rays or snorkel.
Costa Maya is where you relax at a "beach bum town" and drink cheap beer.
The last time we were at Costa Maya was a week before Hurricane Dean....we spent the day at Chac Chi. But it has not rebuilt since the hurricane...so I don't know where we will go next month when we return.