I am still not sure NEXT is the way to go.
For example, let's say I want iPhone 6+ 64GB. I can get it now for $399 or $42.45/month (Next 12)
Let's say I go with Next, and 12 months from now I want to get iPhone 6S. At at point, I would have paid (12x42.45 = $509.40) for a phone I have to return to AT&T (trade-in) in order to get the new one, and then start paying for the other one, etc.
If I just spend the $399 now, I could sell it in 12 months, and use that money towards the new phone, which will be at full price since I wouldn't be eligible to upgrade but it would probably cover around half the price (~$300).
So if you always upgrade when a new phone is available, it still seems to me NEXT is more money than the 2-yr contract.
Only that you are not required to return/trade-in your phone after 12 months when you want to upgrade. You can pay off the remaining price of the phone and it is yours. They will then even unlock it and you can sell it as an unlocked phone which will yield a better price.