We did 10 days there in April, 3 on St. Thomas and 7 on Virgin Gorda. Dove 2-3 dives every day. I'd have to say that the diving was better in the BVI's, especially in the Dogs, Norman and Cooper Islands area. Both areas had a profusion of soft coral and fans, but surprisingly little fish life. But on just about every dive we saw sharks, turtles, barracuda or other interesting marine life. Best dives were the Indians off Norman Island and the Ledges or Chimney off the Dogs. Aquarium off VG was the best for fishlife, it's a shallow 2nd dive on the way back in. And there's the Rhone, the BVI signature wreck. It's the most interesting, intact 1800's wreck I've ever seen. From the BVI's it's 45min, from St. Thomas it's a day trip, 2-3 hrs. each way plus customs clearance.
Visibility on both islands was in the 50-60' range, maybe up to 100' in some areas. Pretty much no shorediving anywhere, on STT there's Coki Beach which would be good for getting wet only, on VG there's really no shorediving to speak of. Although there is excellent snorkeling at the Baths - something you've got to see if you go there.
We dove with DiveIn on St. Thomas - at Sapphire Beach resort, which looked pretty nice - though we didn't stay there. Good outfit, fast boat, they limit the divers to 12 on a boat that could take 16 so it's not crowded.
On BVI we dove with DiveBVI. I can't say enough good things about them. We had 10 divers so we had our own boat, their service level was exceptional. I'm sure you've seen ads for diveops where they claim that you'll only touch your gear getting it to the boat, in this case it was absolutely true. I've never seen 3 people work so efficiently in setting up gear, changing tanks, and being ready to dive with the group as needed. They have a new custom Newton diveboat. At the end of the week, our gear was cleaned thoroughly - around our schedule not theirs - before pickup. I did do several afternoon boat dives with other divers and different members of their staff and everyone provided exceptional, friendly service - so it wasn't just for our group. If I ever go back there again, they'll be my only choice. They also have 3-4 locations around VG and Marina Cay so it's convenient wherever you stay.
If you want a quiet island, Virgin Gorda is it. Driving there is pretty intense, lots of serious elevation changes. Get the car with the most torque if you rent one. And check the brakes...
More people/nightlife would be Tortola. On the US side, St. John looked really quiet, St. Thomas gets all the pod people(cruisers) so it's a lot of hustle/bustle with traffic everyday. Red Hook on the East End there moves a lot slower though and you actually get to experience the local culture there. Dive prices were a little high on both places, but everything is imported so their costs are higher.
PM if you need any more info.