The 16 GB 6S is $388.92/year on Apple's plan, $260.04/year on AT&T's Next, and impossible to calculate on T-Mobile's JOD (since T-Mobile don't provide a calculator and can adjust trade-in values on a whim).
If you have the money to spare, the Apple program is the most hassle free, just also the most expensive.
First, Apple includes AppleCare+. Valuing that is a bit difficult, since it is normally $100 but that gives you two years of benefits (including a warranty for year two), and for Apple's plan you only keep the phone for one year. But it's convenient that if you drop your phone in the first year, it's only $100 to replace it.
Second, if you want to trade the phone in at 12 months, you need to do AT&T Next 12, which is $32.50 per month, or a total of $390. So it's about even.
My take is that Apple's plan is better if you really plan to trade it in after 12 months, or if you would have gotten AppleCare+ anyway. If you might keep the phone for longer, and don't care about AppleCare+, AT&T is probably the better choice.
Next upgrade, I will either be buying outright or using the Apple plan. AT&T Next was a disaster for me.
When you purchase an AT&T Next upgrade via Apple, an email is sent from Apple with forms and instructions for returning your old phone. For one of my phones, I received this documentation. For the other, it was nowhere to be found. The link on Apple's site to resend the email didn't do anything.
First I talked to Apple. They said talk to AT&T. People at AT&T didn't even know why I was trying to return my phone. Basically, they didn't even know what AT&T Next is. I finally got someone at Apple who immediately said "don't talk to AT&T; we'll take care of this." And he tried. Even though the email comes from Apple, it hits a backend AT&T system. And it was AT&T's system that was stuck. All said and done, it took me a month to get my form, just so I could actually send the phone back.
And then one of my fears was realized. Not having received the phone in a timely manner, AT&T charged my account for not sending it back. That took another month to resolve.
I wasted hours and hours trying to fix this and it was one of the worst consumer experiences of my life.
They forgot to bill us for the first nine payments (got a letter saying an error had occurred and we wouldn't be responsible for them), so I've been happy with it thus far. :-)
I was able to trade my girlfriend's iPhone in on the Next plan at a Best Buy Mobile store without any issue. All the documentation and return was handled in store.
AT&T Next 12 is $32.50/month + $15 activation, compared to Apple's plan at $32.41/month (including AppleCare). If you never use AppleCare, then the two plans are pretty much the same. If you do use AppleCare, Apple's plan is ~$129/year cheaper (the price of buying AppleCare every year).
The other advantage to Apple's plan is that they give you an unlocked iPhone. AT&T's are locked until you pay the phone off (which is never if you're trading in every 12 months).
I was hoping Google was going to offer a similar program. I have Jump with T-Mobile and would like to get the latest Nexus every year but it they don't sell them through carriers then Jump sucks.