I live in the Sonoran Desert and the 6A screen just wasn't bright enough when driving with sun glasses on. I upgraded to 7 with it's essentially Samsung level of brightness (about 20% brighter than 6). Perhaps a weird use case something to consider. The 6/7 wireless charging is also a nice to have.
The brightness is not something I could have figured out from "specs", nor is it more than cursorily mentioned in reviews.
On the + side for the 6A, I did prefer the slightly smaller size.
In either case, I LOVE the Caseology Athlex case for grippiness and screen lip protection for either phone.
I would go with the used 6 myself, but VERIFY that it's Boot Loader Unlockable or you can learn it the hard way like I did.