I was m\not aware you could choose towers on ST, I thought they put you on the towers they wanted.
The Straight Talk BYOD package includes SIM cards for AT&T, Verizon, and T-Mobile. You choose. I'm not sure how it works with Sprint, but I do know that Straight Talk also offers Sprint service. Maybe you need to buy a Straight Talk phone designed for Sprint? I do not know for sure.
The AT&T and T-Mobile SIM cards will work in pretty much any unlocked GSM phone. Also, the AT&T SIM will work in an AT&T locked phone, and the T-Mobile SIM will work in a T-Mobile locked phone. The Verizon SIM will work in a Verizon LTE phone, as well as a few factory unlocked phones that work on Verizon (this does not apply to you since the factory unlocked phones that work on Verizon are all iPhone or Android).
As far as data speed, as far as i know you will get full LTE speed. You should get about the same download speed as you'd get with a plan directly from the carrier.