Everyone seemed to jump (heh) on this being about the Jump program, but T-Mobile does offer a regular monthly payment plan in addition to the Jump option. In the case of the EIP (equipment installment plan) without Jump, you can pay off the phone at any time to start a new EIP with a new phone. The phone is yours to sell (you can use a site like Swappa to make it easier). The only issue is you need to have the ~$325 cash available to pay off the phone since you need to do so before you sell the device. If you're planning on going with the iPhone this is probably your best bet. To do the math for the 16GB 5S...
Original price: $648
Monthly payment: $27
With Jump, you would end up paying $27 * 24 (cost of the monthly payment over two years) + $10 * 24(cost of the Jump program for two years) = $888 every 2 years to use Jump to get a new iPhone every year.
The 16GB T-Mobile iPhone 5S is still going for $420 on Swappa. Assuming that will continue to drop once the 6 is actually out, let's go with $400. Without Jump you have $27 * 24 + $324 (cost to pay off first iphone) - $400 (sale of first iPhone) = $500. In both cases you end up with a phone that is half paid off. Even if you want insurance, which is included in Jump, Applecare is only $99 for two years so you end up at $599 vs $888.