Based on the website (http://www.att.com/shop/wireless/next.html), they lay it out the payments assuming you buy a Galaxy S5 (which I assume is similar in terms of pricing to the iPhone 6).
Right off the bat, you know that the 1 year plan isn't really saving you anything considering you nearly paid for more than half of the phone. Which brings me to this:
Doing some math and skipping the boring algebra, I came up to that doing 18 months Next Plan and using the 24month option to pay it off so that you can keep is actually the best deal! Based off of their 1 year comparisons, the 18month plan is best but who would want to give BACK the phone at this point? If you were to use the Next plan of 18months and 24months to pay it off, you would be saving yourself around $190.
Here's the breakdown:
