AT&T made significant changes to their plans earlier this year, so buying a 2-year contract from them is now a terrible idea. You're much better using their installment plan or just paying full price for the phone.
http://www.att.com/media/att/2014/support/images/MSV_Image_3...
With a 10GB plan you end up paying $800 for the phone ($199 up-front, and $25mo extra) under the 2 year commitment plan. Definitely a better deal to just pay $650 up-front.
In any case, I'm with Aio Wireless, now Cricket Wireless -- same network, but much better deal, plus unlimited everything.
Scroll down to family plans. Now a new line just costs $15 if its a phone not on a contract. Else its $40