I've not been to Sharm itself, but did a live-aboard trip out of Hurghada with Colona Divers, and I go to Cozumel every year, so I think I have at least some perspective on this. I'd say that the Red Sea diving compared favorably to Coz, but Cozumel still wins in the end. My understanding is that the Ras Mohammed Marine Park around Sharm el Sheikh is quite nice, so I wouldn't mind going back to Egypt at least one more time to check it out. There's also quite a bit of diving activity around Dahab, which is a bit further up the Sinai. As I said, I went out of Hurghada, which is located on the east coast of Egypt, about 3 hours south of Suez by car. There are also flights directly to Hurghada, because it's somewhat of a resort town with lots of tourist class hotels.
Anyway, my impression was that Cozumel has somewhat more abundant marine life, at least in terms of the larger critters such as turtles, rays, etc. however with a lot of the Cozumel reefs currently damaged by the two recent hurricanes, I'd expect that Red Sea reefs to probably be in somewhat better shape, at leat in the near term. I'm sure Coz will recover soon enough, though. I will second the suggestion above to consider a liveaboard, because that's the best way to see the best reefs. I remember our last dive back in Hurghada before we returned to port (site called "Aquarium"), the reef was half dead and had about 7 dive boats on it with divers milling everywhere, however the reefs that we saw further south were very nice, and we were the only divers there. I was pretty happy with Colona Divers as well. They are a scandinavian op and are located at the Hotel Magawish resort. For our liveaboard safari, we had only 6 divers and 2 divemasters, so it was a very comfortable and enjoyable experience. I'd go with them again if I had the chance.