I have numerous dives with both Deep Blue and Blue Magic. Most of my dives with Blue Magic were with its original owner, but a dozen or so were with the current owner. Some of the personnel, particularly divemaster Enrique Ascencio (Pumbaa) were there the entire time.
Blue Magic was a bit fancier with Joyce, the original owner. Warm, dry towels after each dive, nice snacks, etc. We had a great time but the sole reason we moved from Deep Blue was that our friend Sandro moved there to be the manager. When the business was sold and Sandro left, some of the perks stopped (such as the towels) and Sandro eventually moved back to Deep Blue. This was nothing about Deep Blue vs. Blue Magic and everything to do with our friend.
We again mostly dive with Deep Blue. You can find recent discussions about them which for the sake of brevity I won't repeat here. They're just fine - a fairly representative, well-established Cozumel dive shop without a ton of frills. All their boats are 6-pax and they have some great captains and DM's, all of which is also true of Blue Magic.
Honestly, if I didn't already have this longstanding friendship, I'd just sign up to dive with Blue XT Sea. The only reason we ever went with Deep Blue in the first place was that Christi couldn't accommodate us and helped us find another place on short notice. I've never managed to dive with her shop but have always wanted to.
Cozumel has few lousy dive shops. Most of them are pretty darned good. Pick one of the ones mentioned here and you won't go wrong.
After the first week of August, it opens up significantly 
