With the Apple upgrade program your phone is unlocked so you can switch to T-Mobile whenever you like. Unlike Jump your phone is locked. You can just pay off your phone if you like and sell it and start over again with the Apple upgrade program again. One thing to be on the look out for is T-Mobile sometimes runs all kinds of specials during new phone launches that could benefit you.
I switched from ATT to T-Mobile and have not been happier. Best move ever.
I second the promos T-Mobile may be running. I currently receive a $12.50 monthly credit and end up paying $5.42 for my device payments. Most people who received a launch day X still have $500 or more left to pay off, where as I have $250.