PSA from AppleInsider

If you're a U.S. customer, you plan on upgrading your iPhone in less than two years, and you aren't interested in T-Mobile, Apple's iPhone Upgrade Program is your best bet, hands down. Carrier installment plans are too restrictive and confusing, while Apple's offering is simple, streamlined, and will let you get a new iPhone every year at no extra cost to you, outside of the ongoing monthly payment plan.