Regarding NEXT, the trick is you have to pay for the phone in full -- by completing the full contract, or paying it off early -- so that you own the phone. If you do, you're getting the phone for the same price as if you had paid full price up front, and if you want to upgrade you can pay it off, sell it, then use the sale proceeds to offset the purchase a new phone, either on NEXT or at full price.
If you trade the phone in for an early upgrade on NEXT, though, it's not a great deal. By the time you're eligible for an upgrade you've paid most of the price of the phone, but you don't own it -- you have to trade the old phone in to get the upgrade, but you don't get anything for it -- the new NEXT contract starts at the full price and term for the new phone.