I live in ATL and fly Delta a lot. If you are using points, not sure how much the difference in cost means as it typically doesn't change much until the 4-5 months leading up to your travel dates, at which point it only goes up. I would watch the seats if you're set on first class as the price/points will go up as first class fills. There is not much value in waiting once you know your dates when it comes to Delta. Prices rarely come down a significant amount to STT. SJU has more competition and occasionally I have found a better deal there by waiting but more often find myself upset that I didn't take the earlier price.

Regarding where to fly into, I would want to know your plans for the evening. If you are trying to get in and get out of the marina that night, I would fly into SJU and take the soonest flight to EIS or arrange a private charter with someone like Fly BVI (my typical plan when I want to get on the water the same day). Delta's flights from Houston (IAH or Hobby) both connect through ATL and you will get to SJU about an hour earlier than STT as it stands now for July 2018, so it really doesn't matter much if you don't have anywhere to be that night.

Going in to STT and taking the 4:00ish ferry would be cheaper if time is not of the essence and you don't mind the ferry ride. Also would depend slightly on where your final destination on Tortola is located.