What cell phone carrier should I get for the iPhone 6 plus?

Right now I have an att unlocked iPhone5 for tmobile that I'm trying to sell. I want a single line that offers the iphone 6 plus for cheap so tmobile is out of the 4 carriers. I plan on joining the Air Force so I may be anywhere in the USA or in the world.
1) Are there carriers that offer military support (also so I don't waste paying for a plan that doesn't work)
and 2) can I get any discounts?
3) I'm leaning toward Verizon probably bc I can sell my phone then only pay a little to buy the new one and then only pay monthly. But the problem is, I hear that verizon uses cdma not gsm, so I'm not sure how great the international support is. But if I'm the USA the whole time, Verizon will be pretty good so it's a toss up.
Help me validate if this is all true and which carrier you think would be best. I'm thinking sprint is my second choice.

Walking through it, I think Verizon does $60 a month and they'll take my old phone for $200 but I think I'll just sell it BC that's really low.
Att has some next program or a 2 year. I tried looking up the pricing for it bc you have to pay for the phone every month plus the plan but I couldn't find the plan prices anywhere, i don't know their website is dumb.
Sprint has $50 unlimited a month with i think $0 down for the new iphone that I forgot if you have to pay off.
All these carriers have changed so much recently I'm a little lost what's even good anymore. Some people use Walmart even for them but I'm not sure how the plans work for those.

Verizon