docmartin:i respectfully disagree having done both (6 x Cozumel and 1 x Cancun). Cancun has some very good dive sites including a wreck that's much better than Coz's. it also has more fish life overall. the "reefscape" is admittedly more exciting in Coz. also, Coz is a much nicer place topside than Cancun.
The "wreck" in Cozumel is a joke, of course. You've got the wild currents in Coz and the deep reefs of Palancar, Columbia, Punta Sur are breathtaking.
I love the Tortuga dive at PDC, but Cancun diving is ho-hum to me. Might as well dive Florida.