I think it depends on the type of mobile plan you have. Like Marrionette mentioned they are going out of their way to make the two year plans less attractive. We have a mobile share plan and if I'd gone with 2yr I would have lost the $25 discount. Whether you go 2yr or Next, you are still paying the same price for the phone, it's just a matter of how you want to divide those payments and if you want to put money down up front or not.
With Next you only pay the taxes up front, no activation fee or down payment, I think of it as a lease to own kind of plan. I chose the 24 month plan with an option to upgrade at 18months. The early upgrade is where they get you because you're trading in the phone you've been paying on but that works for a lot of people who want the latest and greatest. At the end of two years you own the phone like you would on contract. The upside for me is I can choose to pay the phone off any time I want, my bill goes down, the phone is mine and I'm free to do what I want with no penalties, I still have my plan or I can walk away. We recently came into some "found" money and that's just what I did, paid it off like I would a credit card or other bill.
IMO, if you can resist the early upgrade trap there are no downsides.