They don't differentiate between the two. The only time you would have to pay for a different plan is if you were going from a "dumb-phone" to a smart phone.
Well my contract is up in December, and currently I am under my parents plan...Once the contract is up I am going to have to buy my own plan...But my dad; working for NASA receives a 15% discount on phones, so is there anyway to use that to my advantage?
*edit* At the moment I am under an unlimited data plan(3g).