The iPhone Upgrade Program is just easier to just ship it back to them or take it to the store to close out the old loan but each upgrade basically is a new loan that can impact your credit score. The return iPhone must be in excellent physical condition or else you'll will need to pay to repair the device using its already included AppleCare deductible. AppleCare is required for all IUP. You do need to get preapproved for each upgrade and pay the remaining to meet at least 12 months payment for the upgrade. If you upgrade, its basically equivalent to loaning or renting the device with incidental coverage. It may get you to upgrade each year by paying about the same amount or just a little higher.
Paying with credit card give you utmost control with no obligation just to resell the device at or higher than trade in value or at resell value towards a new iPhone. It just the hassle to sell and if you're trying to sell it at its highest value possible. If you purchase AppleCare, you can get a prorated refund depending on the usage.
I pay with credit card using Barclays Apple Reward to finance the purchase while giving me the control to resell it or trade it in depending what is the resell or trade in value.