Hi Sweet Sue,
I'm baffled as to how you selected these two to compare out of the about 100 dive ops active on Coz. They are quite different.
Scuba Du is hotel-based, with the original shop at the El Presidente Intercontinental (south) & a newer one at the El Cozumeleno (north). Aldora has a free-standing shop in town, across from the original hyperbaric chamber.
Aldora draws experienced divers & tries to limit boats to six, while Scuba Du draws a mixed bag, including newbies, and can take as many as 16. Not surprizingly, Scuba Du is a bit more regimented.
Related to the above, Scuba Du dives standard aluminum 80CF tanks, while Aldora dives steel 120CF tanks. As such, Aldora divers have enough gas for long, deep dives & in fact do them. These tanks require that your reg be converted to a high pressure fitting, which Aldora does for you at no cost.
A two tanker with Aldora is $70, while it's around $60 with Scuba Du.
Both have nice boats, although Scuba Du's are nicer, and both have professional, safety-oriented crews.
Nice thing is, either choice you win.
If this reply has raised additional questions, feel free to post them.
Best regards.
DocVikingo