Does verizon exist on STX, or is Sprint and AT&T my only options?
T-Mobile w/ roaming - on AT&Ts network
Verizion maybe be able to roam?
Since you're moving to an area Verizon doesn't service break your contract (should be penalty free) and hook up AT&T or Sprint. AT&T is said to be more secure and reliable on STX.
Version does work but it used to be roaming and very expensive. I switched several years ago so I don't know about current charges.
There's an off chance Verizon might work on the Sprint network, since they're both on a CDMA signal. But they'll probably charge out the nose for it. I bought a Virgin Mobile prepaid for when I move--fully unlimited talk for $50, and it runs on the Sprint network as a home calling area. It's a good option if you're not ready to tie down to a contract just yet.
I had Verizon back in 07, when I was first on island. The service worked, but it was considered roaming & the connection was spotty. I got outta my verizon contract w/o any hassles or fees since they didn't have "normal" service on STX.