Sprint and HTC M9? topic

Whats going on guys?

So with me finally breaking off my family plan with my mom, im getting my own carrier and account. with that i'm planning on getting the M9. Currently i have Verizon, which i dont mind, however, ill be switching carriers i was thinking sprint because as of current deals by me, they will offer to pay the verizon cancellation fees. idk about AT&T if they will do the same, i was gonna price all 3 of them out. anywoot, my main question is this: How is the service with Sprint and AT&T? After i graduate High school, im enlisted with the military which means ill be traveling around a lot. i have no idea where ill be getting stationed at all, but i wanted to make sure i would have service. The M9 is going to be both a phone and a computer for me for a while and i dont want to have to rely on just Wifi. my other question was, if i get stationed out of the US, how would i still be able to use the phone besides wifi?