I can't speak to unlimited plans because I don't have one but other than that how is Next worse than the 2 year contract? Either way you're paying off the phone in installments.
No it's not. At the time I looked at this (when the iPhone 6 came out)
- with Next: you pay the FULL price of the phone over (say) two years. That's it. No subsidy.
- with non-Next: you save $360 on the price of the phone over two years, but then once you pass two years you keep paying an addition $15 per month higher than the "raw" data plan. Where do the savings come from? You pay more for data each month than Next, but Next has an additional "cost of the iphone" item for two years which ends after two years, whereas with this plan the data cost never comes down.
So if you upgrade at exactly two years you save $360. If you delay, every delayed month you are paying ATT an extra $15 and so your subsidy is going down by $15.
[The numbers may be slightly off cause I'm going from memory, but they're approximately correct.]
- with upfront purchase you pay the same as Next, you get an unlocked phone right away (so that's a win) and you don't get the option to sell the phone to ATT at the end of your payments period (but IMHO that option is worthless anyway).
There are also differences if you have family plans, data sharing plans etc. I only looked at MY situation (one iPhone, no sort of sharing plan, and that's what I saw --- 2yr contract is best if you want to upgrade after two years, otherwise buy an unlocked phone, Next is NEVER the right choice).