The upgrade eligibility is not a rebate -- it means your current phone has finally been paid off & you are now free to pay on a new one. All the carriers will charge you the full iPhone retail price over the course of your contract -- you only choose how fast that will be. If you do upgrade, the family bill will increase, which you should pay. I believe only T-mobile has an unlocked version because it uses a different band technology.
Reselling the new phone only works if you can get more than the full retail price, which was possible if you were selling to a country like China during the very first days of the 6 launch. I doubt that's the case now.
During the first month of the 6 launch, most carriers were offering sizable trade-ins for older phones well over their resale value.