I have been to Maui several times and St. Croix once. My trip to St. Croix was only a couple of months ago.
In my opinion either place is great, whichever you choose you will not have made a mistake.
Generally speaking I found that St. Croix is less expensive overall. I am not only talking about the diving, but meals, gasoline, groceries, diving, almost everything. The rental cars on Maui are perhaps a little less expensive but with a little advanced shopping you can probably get good deals on both islands. Hotels or other lodging can range from moderately priced to high dollar on both islands, but after trying a few different places on Maui I like the "Sugar Beach" condos (a little time spent on Google will turn up several condos in this complex). For St. Croix we stayed at a hotel, and frankly I like condos more so next time we will probably look for a condo on the island. With this in mind the two experiences do not really compare in price or the level of the accomodation.
As far as the diving is concerned Maui has lots of colorful fish buth the coral is somewhat bland. St. Croix has some of the best coral I have ever seen, but not as many fish as Maui. The coral reef is very healthy and it runs for miles. On St. Croix all of my diving was on the north side of the island, I was told by some local divers that the west end has more colorful fish but the coral is not as plentiful. Shore diving is available on both islands. If you want to be able to do some deep dives from shore with long but managable swims then St. Croix is probably a better choice as in some locations the wall is only a couple of hundred yards from the beach and it drops to a few thousand feet. From Maui I am not aware of any shore dives with depths below 60 to 80 feet with a reasonalble swim out.
The visibility in Maui is probably a little better, most of my dives seemed to have vis in the 75 foot to 150 foot range, while St. Croix dives tended to be in the 50 foot to 100 foot vis range. Both islands have small creatures, and medium sized creatures. I saw more large stuff on Maui, but it may be only luck as I am told that there are plenty of large turtles and rays around St. Croix, I just didn't see many of them.
Both islands are in the United States, but for some reason if you travel to St. Croix you will need to have your passport to return to home. Another oddity of St. Croix is although the automobiles are standard style american cars where the driver sits on the left, you drive on the left hand side of the road. This is extremely unsetteling at first, but you eventually get used to it. On St. Croix drivers are generally some of the most polite I have ever come across, if anything you need to be looking out for stopped drivers who are allowing others to turn or enter the flow of traffic.
I hope this helps, I plan on returning to both places in the future so you really have a tough choice. I don't have anything bad to say about either place.
Mark Vlahos