full fin,
We learned how to dive with
http://www.divebvi.com/ on Virgin Gorda in 03, dove off St. John in '04, and have been to the BVI a # of times since then. Jeff, AKA "VGdiver" has given you a good description of the differences. We have 11 divers in our extended dive family, and we reached a consensus that we prefer the BVI. Haven't been to STT, so don't really know what the diving is like there. The BVI has a healthy reef system, with a lot of colorful coral and your typical Caribbean reef fish species. It also has a # of wreck dives, including "the wreck of the Rhone," one of the most significant wreck dives in the Caribbean:
http://www.scubaboard.com/forums/lesser-antilles/208262-140th-anniversary-wreck-rhone.html
Personally, VG is probably our favorite island in the Caribbean, and Dive BVI is an outstanding dive shop, with 2 locations on VG. However, if you are going to Tortola, Cane Garden Bay is a very good location, as others have mentioned. Here is a listing of some of the accommodations on CGB:
http://www.b-v-i.com/Tortola/Lodging/CaneGardenBay/cane.htm
This is also helpful:
http://www.turq.com/bvi/tortola/placestostayindex.php
I would also encourage you to take a close look at Chateau Relaxeau Caribe on Little Apple Bay, just a short drive W of CGB. You are near several very good restaurants, close to Smuggler's Cove, probably the best beach on Tortola, and will not have to deal with cruise ship passengers on CGB, their favorite destination on Tortola (depending on when you will be on Tortola):
http://www.britishvirginbeer.com/chateau_relaxeau_caribe_2006.htm
If you stay on the N side of Tortola, then I would give serious consideration to catching the morning ferry at Soper's Hole, AKA "West End," to JVD in order to do 2 or 3 dives with Capt. Colin of
http://www.jostvandykescuba.com/
There are seldom dived, pristine sites at JVD, including Playground, a very colorful site with caves, swim throughs and large schools of fish (tell Colin you want to do your SI on Sandy Spit, an acre or so sandy patch that is a terrific place to spend an hour), Cathedral, a swim through that leads you to a cave inside JVD in which you can surface, and Pirate's Purchase, a fully intact sail boat in around 90' of water that Colin "discovered" a few yrs ago.
You can see photo albums of all of these places, including the dives, here:
http://honeymoon2.smugmug.com/
I'll be on VG next week!
Enjoy your trip, alashas