Straight Talk used to re-sell pre-paid, month-by-month AT&T or T-Mobile service cheaper than you could get either directly from the carrier. No idea why/how. Cancelled T-Mobile plan for ST/T-Mobile and used them for a couple of years, at least.

The signal strength was identical; used the exact same towers. It'd use a different APN - but that only affects data traffic. No practical difference, other than needing to remember to re-enter the APN info if you wiped/reset the phone.

Switched to Google Fi long ago; it's an MVNO that uses whichever of Sprint, T-Mobile, U.S. Cellular, and Three (Hutchinson) towers are in-range/usable at the moment, switches seamlessly between them. If it can't get mobile, it'll use WiFi (public via a VPN or otherwise).

Pretty convenient to get off a plane in any one of ~170 countries and have your phone Just Work too. With the same prices as if you were sitting in your home.

You do need a Fi-compatible phone; I just replaced a damaged Motorola Nexus 6 with a brand-new one for $140. And bought a spare :)