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.
If you upgrade every year, this is an interest free loan with Apple Care+. Just do it and make sure to order this Saturday for a delivery by the 25th of this month! Also, iOS 9 is out on the 16th.