Here's my $.02:

Timing: some of this will depend on availability. This is high season in the Caribbean. I don't think any of these places get crazy crowded until Spring Break starts in the US & Canada.

I have not been to Cayman or Curacao.

Here's why St. Thomas is a good choice: multiple pools & hotel options (MVC villas, Frenchman's Reef, Morning Star) listed in my preferred place to stay. The villas include full kitchens & living room. Multiple restaurants on-site, including the 4-5 star Havana Blue. Downtown STT has many great restaurants, too. There is an $6 water taxi into town. It does stop at 5PM(?). For romantic beaches, take a day trip via the ferry to St. Johns. Many world class beaches: Trunk Bay is our favorite. Take wine, cheese, crackers & towels with you (no glass bottles) for a incredible day. You can also snorkel at these beaches. You can rent a car, but be aware they drive on the left hand side of the road. If you're inexperienced doing that, you might want to take cabs. Yeah, it's pricey but for us much more relaxing (and cocktail intake isn't diminished). Lots of spots on the island of STT to visit: Maegens Bay, Water Island (a great grocery store near the water taxi to this island with great food & alcohol). Probably the biggest downside is the cost: Food & cabs are expensive. But, a trip to the grocery & liquor store will help. Flights are reasonable to get here. Also, you can get boat charters to take you to any BVI you want: Jost Von Dyke, Virgin Gorda, etc. Or you can get to most of these by ferry.

St. Kitts: We're returning here in April for the 2nd time. Much less developed than STT & Aruba. Which is not a bad thing, just a comment. The Marriott itself is I think the best in the Caribbean: Hotel & MVC villas. Multiple large pools & multiple on-site restaurants & bars. Our 1st trip was an all-inclusive but this is no longer available here It was well worth the points to get it. We'll be in a villa so we'll be doing more eating in this time. Also, buying alcohol offsite is a much cheaper way to go. There's a decent beach, no waves but very calm & relaxing. We toured the island last time via taxi for 3-4 hours which I think cost $100/pp w/ tip. We didn't rent a car. It's harder to fly to/from the US & more expensive. We plan to do more island exploring this trip & will also include a trip to Nevis via ferry. I can't comment on snorkling here but I'm sure it's all around. We watched a local catch lobsters w/ a spear while we were on the hotel beach last year. Overall, a very laid back uncrowded island (for now). It's starting to get more built up.

We love both places.

We also like Aruba but prefer the Renaissance suites as they are usually less crowded than the other end of the island. Plus we can walk to the numerous restaurants downtown.