Both iPhone SE models work the same on AT&T and T-Mobile. One has support for Sprint's high frequency band and the other has support for Verizon's low frequency band.
Honestly, the non-Sprint model is the one to get unless you travel out of the country a lot. Missing the high frequency band on Sprint isn't going to give you worse service, but missing Verizon's low frequency band will. And if you ever roam on Verizon or there's even a slight possibility you'd ever use it on Verizon, you don't want to miss out on better Verizon coverage.
I bought an iPhone SE to use on RingPlus (uses Sprint's network) and I bought the AT&T model full price and unlocked, since it was the only one available at the time. Works great on Sprint.