In my opinion coverage is way better than postpaid AT&T. I travel down to Iowa as my fiance's parents live there (I'm 2 hours west of the twin cities in minnesota so it's about a 350 mile drive along the interstate.)
When I had AT&T I would get down into Iowa and as soon as I got off of interstate 35 I'd be off network and have no data (because they were jerks about not telling you when you were off network.) But on Straight Talk, In the same locations I wasn't off network (was home) and had access to data and talk/text.
ST also has contracts with regional GSM carriers... so while you may only get edge service, at least you'll have service!
I buy broken/used iPhones (4 & 4S) on any carrier! PM me info about your phone and your asking price and I'll get back to you!