A lot of people have been confused about AT&T's Mobile Share Value plans, including me, so I did some research.
Here is AT&T's caculator for comparing plans for getting a new Iphone 6.
You can see that the Next 18, for most people, is the cheapest way to get a new Iphone 6 from AT&T.
A lot of people are upset because they didn't read the info about buying the phone for $199 (2 year contract) then finding out they lost the Mobile Share Value discount and their plan had increased $15 to $25. I believe Verizon is priced the same way, but not positive.
You can still take your old Iphone 5 in for a credit of $200 to $300 dollars depending on model.
AT&T calculator This may explain it better.
AT&T vanishing discount, don't get burned.