My wife and I loved St Martin and we stayed on The french side so it was a little bit of a different atmosphere but we would definitely go back. We were disappointed a little with St Thomas because we just didn't find it as nice as St Martin overall. We also went to San Andres but it could be difficult to get there as you would most likely have to go through Panama or Colombia and the island is so small that I would recommend checking out somewhere St Martin first.
If you are travelling from the west coast, going to Mexico such as Cozumel or Cancun would cost a lot less and save some travel time. Have fun wherever you go!!