You need to decide what is important. Here are a few things you need to consider when selecting a dive operation. Cost- Obviously, but don't forget you often get what you pay for. There are other things that are more inportant to consider besides price.
1. Tank Size. You may want to pay for getting a bigger tank. The standard tank size is 80cf, other tank sizes used by dive ops are 90, 100 and 120 cf. I know of three dive operations that use Steel 120cf as their standard tank size. Aldora, Living Underwater, and Liquid blue . Other dive ops have 80cf tanks and offer 100cf as an extra charge. Diving with 120cf will give you a 50% more gas making a longer dive. We normally get 80-90 min dive when we dive with 120cf tank.
2. Speed/size of boat(s), number of divers on board....and if the op has more than 1 boat. The smaller fast boats get you to the Southern sites quickly (unless you are staying at one of the three Southern Hotels). Big slow boats usually mean larger number of divers on board and a looong day.
Cozumel limits 8 divers to 1 Dive Master (DM). However larger "cattle" boats put 2 or or more DM in the water depending on boat size and number of divers on the boat. Some of the cattle boats can have 16 or more divers in the water. They even advertise "No more than 8 divers per group", but they don't say how many groups per boat. They often put a DM in front and one or two on back with a swarm of divers in the middle.
Some intermediate size boats (still fast) will have 2 DM and 10 divers.
Some dive ops have multiple dive boats they can group the move advanced divers together so you are not mixed with new divers who suck their air and force the group to surface. (Not an insult but new divers but they often do no it have proper buoyancy control, seem to like swimming at high speed, and want to be everywhere and see everything).
Most of the boutique dive shops limit 6 - 8 divers per boat, one DM and a boat Capitan. I prefer fast 6 packs or 8 packs because I like to get to the reefs fast and small dive size.
3. Surface interval and location. Beach, beach club, or boat and what snacks and drinks are provided.
4. Departure location and time. What time does the dive op departs and from where? Do they depart from the downtown dock, pick up at your resort dock, , leave from dive shop, pick up at your hotel? Depart 7:30, 8:00, 9:00 and as late as 10:00. I be leary of the after 9:00 depart because that may be a Red flag indicating you may be with Cruise ship passengers (they tend to be newer divers). I like leaving early so I get back in my condo by 1.
5. Boutique (Full service or not). Do they keep, clean, store and setup your gear if you are diving multiple days?
6. Other: Some dive ops offer fleece Jackets during Ride after dive. Some offer WiFi on board. Do they go to the most Southern reefs or just drop you off at a closer reef to save time and fuel? Do you have input on what site to visit?
So who should I dive with? There are over 80 dive companies on Cozumel. There are dozens of really great dive ops and a dozens more that are ok. I don't believe there are really bad dives ops on Cozumel, they would not last long with all the competition. All I can tell you is a list companies I would dive because what is best for me based upon my preferences and offer boutique experience. They are, in alphabetical order (# boats - divers per boat):
Aldora (5-6 and 1-12), Blue XTSea (2-6), Tres Pelicanos (2-10), Living Underwater (1-8)
Other highly rated dive Ops that offer service and are often highly mentioned as having quailty fast boats and staff are:
Dive with Martin, Dive with Allison, Liquid Blue, Scuba Tony
If you dive with any of The above dive ops, you should have a great experience.
One last note, I tend to stay away from resort dive ops. They tend to be larger cattle boats and often has a set scheduled of dive sites and do not choose sites based upon group ability. Also, all resort except Scuba Club allow other Dive Ops to pickup at there pier.
Now I'll watch the cheerleading!