You can activate the iPad Air 2 or iPad Mini 3 on any network, you just need the proper SIM card, and you have the freedom to change carriers, but will have to change SIM cards. For some reason, the people in the Apple store are not well informed about the interchangeability of the new iPads. There are two kinds of SIMs now which cover all 4 current carriers: the Apple SIM, which can be used on AT&T, Sprint & T-Mobile, and the Verizon which is exclusively for Verizon.
I read up about the features of the new iPads, and the interchangeability of the new iPads is the feature that convinced me to go ahead and spurge on a new one. I bought an "Open Box Buy" iPad Mini 3, and when i trieed to activiatee the cellular, I found that it had an AT&T branded SIM, and I got an "Invalid SIM" message. I made an appointment at the Genius Bar at teh Apple Store, and explained that I wanted to activate on Verison and needed a Verison SIM. The guy who was helping me said he didn't even know you could do that. He went in the back, talked to someone to verify that I could switch carrieers. They gave me the Verion SIM, I activated it and all is well. Read the articles linked below, and stand yuor ground since Apple isnt giving their emplyees the latest info. My theory is that Apple isn't making the interchangeability well known because they don't want to aileante their carriers. (I'm sure Apple willl save a lot of money not having to keep inventory of iPads for each of their carriers, and instead only have to keep 2 different types of SIM cards on hand instead).
http://www.houstonchronicle.com/business/drmac/article/Where-you-buy-an-iPad-Air -2-or-iPad-mini-3-matters-5851049.php
http://www.zagg.com/community/blog/how-does-apple-sim-card-work/