None. They're all pretty much the same. It's whatever voice plan + a data plan on top of that. So you're looking at a monthly phone bill of at least $70.
The best smartphone plan right now is Virgin Mobile. It's $25 a month for unlimited data and text. No contract. Their phone selection kinda sucks. Only 2 Android phones to choose from. But if you mainly want to browse the web, then either should be sufficient. If you're set on getting an iphone, then you gotta pony up the $70 monthly fee and a 2-year contract with ATT or Verizon.