I've taken courses from various agencies i.e.. YMCA: OW, PADI: AOW, RESCUE, DMC, CAVERN, and also SSI for Nitrox. I don't think the agency matters that much (sorry PADI), what does matter is the instructor. My instructor for the Nitrox certification was bad,and I had much better instruction before, and may take another Nitrox course just because my first course was really bad!
CAVE DIVERS DO IT BETTER IN THE DARK!