We are just heading back to Curacao for our 3rd time this Christmas - have done Cozumel 3 times as well.
If you are all about the diving you will love Curacao. Once you get comfortable shore diving and can do it without a guide - the cost of each dive drops to the cost of a tank refill - about $10 if you get a package. Rent a car and go. Not sure if you have seen this
link to dive sites before, but there are a number that have videos and tell you which are shore dives and which are boat, etc. If you want to do a boat dive - I would highly recommend Watamula - one of the best I have been on in the Caribbean. My favorite shore dives - Tugboat, Directors Bay, Playa Kalki. I hear Porto Maria/Cas Abou area is nice and plan to try that this time around along with Carpile.
Also - if you use google maps street view - you can see many of the sites/entry points and the beaches. Many also have dive shops and restaurants on them. You can't go wrong.