Apple's iPhone Upgrade Program?

So Apple now offers "iPhone Upgrade Program" starting with the iPhone 6S, and I want to know more about it before I decide to sign up for it.
From what I've gathered, once you buy the new iPhone directly from Apple, it will be unlocked. The program gives you the option to get a new iPhone every year after 12 installments. Does this mean I will have to pay for 12 monthly costs before I can upgrade it?
My current carrier is AT&T, and I'm eligible for an upgrade. Which would be better for me, getting the new iPhone from AT&T with the AT&T Next or from Apple with the iPhone Upgrade Program?
Also, does AT&T still offer a 2-year contract?
Thank you!
Added (1). I forgot to add, with the iPhone Upgrade Program, will I be paying for the new iPhone price when it comes out? This seems like a bad choice because basically I will be paying for new devices every year.

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