Which Iphone for Ting

I had a very bad experience with ATT and vowed I would live without my phone. It's hard.

Then I found Ting. My son uses their service, his bill is tiny and their customer service have been great. They offer both GSM and CDMA networks. My son bought an unlocked iphone4 and a TING sim, his service in our area is OK but the data service is dreadful. On contacting TING he was told that in our area, CDMA was better.

I want to buy a 6s (which I had before the ATT disaster) using the apple new phone each year scheme but for use with TING, but I don't know what to buy. If I've understood things, CDMA is verizon or sprint, but if I buy a verizon or sprint phone from apple, won't it be locked to that carrier? If I buy an unlocked phone, won't it be GMS only like my sons?

Please advise!