I've been to Cozumel 4 times now and stayed at the Iberostar each time. Dressel may have a lot of people on the boat but their boats were larger than the two I was on when I went with Dive with Martin this past May. DWM will pick you up at the Iberostar pier and you can handle all the paperwork and reservations without taking the taxi back into town. Aldora will pick up at a wade-to location between the Iberostar dock and Alberto's.
Dressel visits the same dive sites as per their weekly schedule, and after a few trips with them I wanted to see other parts of the reef. They are quite safety conscious, dives may be limited to 50 minutes, and if you are paired up with an air hog, there's a chance that they will send you up with your buddy rather than finding someone closer to his/her air consumption. I've had it both ways.
Iberostar means you're close to the major dive sites, Dressel returns to the resort after each dive so you can grab a quick drink and bathroom break.
If you are a "foodie" than this AI isn't for you. After a week I found the lunches to be quite repetitive but I'm a picky eater and don't care much for the salad bar offerings.
The next time I travel to Cozumel, most likely it will be to the Iberostar, on an Apple Vacations trip. I'm not too adventurous when it comes to learning a new place, especially when I don't speak the language and don't know where to go. Too much hustle and bustle in the city for me.
That's my two cents worth. It all depends on what you want.