Assuming pricing doesn't change, at the end of 12 months an iPhone 6S Plus 128GB would cost a total of $570 ($47.50 per month x 12, OR $270 down payment + $23.75 per month). That leaves us with a $380 balance.
At this point, there are two options. Trade the iPhone in to have that $380 balance waived, or sell the iPhone and pay off the balance. Selling it seems to be the much better option, as it should be very easy to get more than $380 for the phone.
Let's assume we sell the phone for $500, that leaves $120 in our pocket after we pay the remaining balance of $380. Subtract the extra $120 from the $570 we paid for the phone over the course of the year, and it the phone ends up costing a total of $380 to use over the year.
It's not as a good of a deal as contract subsidized prices. I've been buying phones at subsidy for $399 + tax every year, then selling them for around the same amount to fund the new phone. With that method, you are pretty much paying nothing to use the phone all year, unless there are hidden fees involved that I haven't known about. Still, I've become accustomed to the amount of my bill, so it seemed normal.
The only reason to go with Next, in my opinion, is if you absolutely want to order via Apple or you don't want to pay anything for the phone right now.
I'm pretty much thinking out loud at this point but hopefully my banter will help someone out.