Default 900a vs 900t questions straightalk

I have straight talk... curious major differences between AT&T vs T-Mobile

also if I went with T-Mobile on straight talk how would I use it on AT&T and would 4G LTE ect or how would it go?

I know I would need to have both AT&T and T-Mobile APN's to switch around on?

Simply put if I got a T-Mobile I can still run all bands on AT&T straight talk but benifit switching APN from AT&T to T-Mobile as well?