Best carrier for iPhone 6s and straighttalk

[dukebound85]

What is the best carrier model to get to use with straightly piggybacking ATT network?

Factors I am considering:
1) Trade in value fro my unlocked 4s or 5s (both 32 GB)
2) all bands being utilized for LTE regardless of carrier
3) If I did have to sign a temporary contract, which carrier has the cheapest overall price for a 6s
4) I want it unlocked

I am wanting a 6s 128 gig

Thanks!