I've been to both and I prefer the diving in Roatan, but Cozumel is so much better topside. IMO, the dive ops in Roatan are a little more customer service and safety focused, dive groups stay together and surface together for the most part (we did have a guy who ran OOA in about 15 minutes and the divemaster took him and his wife up for a safety stop and had them surface alone). In Cozumel, you're pretty much on your own, and I hope you're okay surfacing in buddy pairs and waiting for the boat to find you. In Roatan, you swim along the reef, whereas in Cozumel the divemasters get mad if you try to swim -- they pretty much just float along with the current. In Roatan, there is one decent grocery store and it will cost you $20 to get a cab there, or you can eat bad bar food every night, and I would not feel safe walking around most of it at night. Cozumel has pretty much anything you want in San Miguel - what a great little town, and its safe to walk around at night. I walked back and forth to town every night for dinner, about 20 minutes each way, with no problems.
What exactly are you looking for?