We were in a very similar situation to you - stuck on AT&T network because of location, with iPhone 5's (not 5s's), heavy data users (2+GB each/mo). We were also under contract, so our phones were locked (At&t will probably unlock them after we pay the termination fee, if we ask).
We just switched a couple weeks ago to Net10. $85/mo for 2 phones, w/ unlimited talk/text, and 2.5GB data each (throttled over that) (comes to $91.xx after tax). Not too difficult to do, and no complaints so far. Online tech support was helpful when I had a question. You lose built-in visual voicemail, but download YouMail, and it's even better.
Not as cheap as some other services espoused on here by I.P. Daley and others, but it's another option.