Cabo dive sites have three areas that most shops regularly dive. The local dives are a 10 minute ride from the marina and include Sand Falls, 90' deep, Pelican Rock, Neptune's Finger, Land's End. Dives can have assorted reef fish, eels, sea lions, maybe a shark or sea horse if your lucky. Further out dives are Santa Maria, Blow Hole, Chileno, 30 minute or more boat trip. Rock formations with urchins, eels and assorted reef fish with the possibility large schools of fish. The better dives to my mind are at Cabo Pulmo. Two hour drive from Cabo with a 15 minute boat trip to the dive site. Some corals but nothing compared to Cabo. I have seen more fish on one dive at Cabo Pulmo than any other dive. Had the good fortune to see a bull shark last summer. Best time to dive is July to November for visibility.
Coz...well there is a reason why divers return year in and year out. The coral formations, even after Wilma damage, are amazing. There is also diversity in the terrain of the dive sites and opportunities to seek out a wide variety of sea creatures and corals. January is my usual time to visit so the previous comment is not necessarily from experience. In 10 days mid January, the port was closed only once. Visibility was 80' to 100' and the day after the port was closed I had the joy of diving with 29 eagle rays. AWESOME!! Most 2 tank dives may take 5+ hours but the SI's are longer as the first dive is usually deeper than at Cabo.
Diving in Coz in the winter and Cabo in the summer works well for me as I hate Caribbean humidity in the summer and both are very convenient to get to from Colorado!