It is correct that the best dives in St. Thomas are wrecks, but having visited both, I would choose St. Croix for the better boat diving and shore diving.
Anchor Dive Center runs an excellent operation in Salt River. Their boat rides are five minutes from the best dives on the North Shore. If you purchase a package with them, you get free nitrox or free air cylinders for shore diving.
I would add that the Frederiksted Pier is one of the best dives in the Virgin Islands, not to be missed. On that dive, we have seen numerous octopus, squid, scorpionfish, lobsters, eels, seahorses, etc. Nothing large, but everything else you can imagine.