You have to also factor in what you could sell the phone for if you went the normal upgrade route. I know you said you didn't want to worry about finding a buyer but to get an accurate cost difference the selling price needs to be factored in. With Next you pay $360/yr and give the phone back for a new one then pay $360/yr again and again. Let's say I have a 32gb iPhone 5 that I sell for $450 because I want a 32gb 5s. An early upgrade will cost $550 and when you subtract the $450 I got for my 5 the cost of the 5s becomes $100 which is much cheaper than Next. Next is a massive ripoff. If you want to upgrade to the new iPhone every year then take the time to sell your current iPhone and do an early upgrade.
There's one thing that kind of gets lost in this math, at least for the 1st 12 months, the initial down payment. Sure, you can sell the iPhone 5 for $450 (assuming 32gb), but you originally spent $299 for it at the start.
Standard route compared with $299 32gig 5 and 5s:
Year 1: $299 downpayment for iPhone5 32gb and hope to get the $36 fee waived. 12 months later you sell it for $450 to pay for 5s at $550, early upgrade. The grand total spent over 12months is $399
Year 2: You'll finally see the benefit of this after not having a downpayment. So it might really only cost $100 after selling a 5s to get the 6 with an early upgrade. Years 1-2 total: $499
After 3 years: You are now on a 4th phone and only spent $599. That's if everything went smoothly to sell your phone, got top dollar before another release, possibly having to using an old phone for a month while waiting for the next model, etc. So probably not smooth sailing.
Next route:
Year 1: Start $27* a month (don't know price for 32gb Next), comes to $324 after 12months. Choose to get a new phone. So in a way, a bit cheaper this time.
Year 2: You've spend another $324 and are now on a 3rd phone. But now you really are paying more over. Years 1-2 total: 648
After 3 years you are now on a 4th phone and spent $972, and even more if we knew the 32gig price. But luckily, it was super convient, no having to time with market to get the best price, shady ebay and craigslist to sell it, etc.
And of course you can always pay Next off early, say a bonus check, while your service is still active and no longer tied to a contract (the installments). So helps if you don't want to pay up front or feel your time is worth more than the hassle with the cheaper way.