how easy to get from STX to STT for a visit?

https://www.seaborneairlines.com/

Or the ferry
http://www.qe4ferry.com/

Those sre the only public options. Private charters available from other airlines.

What about Cape Air and Seaflight?

Thanks for all the great options

Yes highly suggest you pop over and stay a couple nights. Cheap places on AirBB - my buddy has one at sapphire beach. Make SURE you get out of town - where the ferry and seaplane land and go to the north and southside and explore. Otherwise you'll just think its crowded. I would suggest the seaplane. If you look at rates now you can find the special deals for the future of $49 or $69 each way. The ferry takes 2.5 hours and is $100. I always take the seaplane for $100-$140 - its 20 min.

Cape Air is the cheapest. The trick is to book it no more than 3 days advance. They slash all their fares three days before the travel date. $76 from STX to STT. $79 on the return. For a total of $155. By far the cheapest. $20 extra for a 50 lb suitcase. No weight limit for carryons. www.capeair.com

Seaflight is $180 round trip, and leaves every other hour, on the hour. Leaves STX on even numbers, and STT on odd numbers. They only allow 30 lb. 714-3000

Seaborne Airlines has special deals for the summer, but not all dates. You can find some great deals. www.seaborneairlines.com

You can take Seaborne from the airport of from the Christiansted dock. There are other small carriers like Cape Air, but they are not substantially priced different nor convenient. They often cancel unfilled flights, which Seaborne will not do.