Not true.
I don't think anyone has mentioned this yet, but If you do Next, and pay in installments, those installments will be added to your cell phone bill. That means those annoying federal, state and local taxes will be charged on your installment payments EVERY MONTH even though you already paid sales tax on the price of the phone when you sign up for Next. You are being double taxed. So Next is not a 0% loan. The monthly cell phone bill taxes are your interest, essentially.
Therefore, if you do Next and pay it off immediately, it is cheaper than carrying out the installment plan (not sure how much, someone want to do the math?)
I have come to the conclusion that the cheapest way to upgrade is to either:
(1): Do Next and pay off immediately.
(2): By a full retail price from AT&T (if you bring from outside (apple), they charge $36 activation fee) AT&T's retail price device IS unlocked, so no difference there.