AT&T: Buying iPhone from Walmart or Bestbuy guide
Upgrading through AT&T is the worst option: they charge sales tax; and they have 30 months upgrade program, 24 months cost $5 more.
Bestbuy: they allow you to do a trade-in(must pick up in-store); they do not charge sales tax; trade-in program can be 24 months no extra fee.
Walmart: provides $50 discount on all new iphones, they do not charge sales tax; train-in program can be 24 months, as well as 30% downpayment can be made to reduce monthly payments.
So there you have it, please chose the best option for you.