I am on a legacy unlimited data family plan as well as is my wife. We spend $145 a month total, and now unlimited data gives you 22 gigs per line before the possibility of throttling. If I switched to Next, I would need an almost $400 plan to be comparable. Sorry, but Next does not save me money.
And if you want to compare apples to apples (I'm sorry, the pun was too easy) under Next's 24 month installment plan you have to pay for 18 months before you can even trade the phone in and upgrade. Under the new iPhone Upgrade program, you only pay for 12 before you upgrade. You therefore are paying much less under that plan than for the Next plan.
I just have to disagree about Next. I have yet to find a scenario that will save me money under that plan vs. every other option I currently have. If I had a mobile share plan, then I'd go with the Apple Upgrade program. Being as I have unlimited data, I'll stick with my current plan as long as possible and continue to pay less for the device than I would with the Next program.
As far as being on an unlimited plan you are right. I'm on a mobile share 30 GB plan back when they did 15 for the price of 30, so we only pay 130 for that shared data. On mobile share, next does indeed save you money.
As far as the upgrade is concerned. Both apple and ATT make you give back the original phone before trading in. Its not as if after 12 months of payments you get the other phone too. With ATT, if you want to keep it, you just pay off the remaining balance. Which is something you can do at anytime, regardless of whether you chose the 12, 18 or 24 month plan. So each year, I pay off the difference, upgrade at 0 down, and sell my old phone for more than it cost me to pay off the difference.