Apparently you can downgrade from 9.1 beta to 9.0 or 9.0.1 without losing data.
https://www.reddit.com/r/iphone/comments/3mbff4/psa_if_on_ios_91_beta_and_getting_iphone_6s/
Does the 6s come with 9.0 or 9.0.1?
I'm inclined to keep her on 9.1 beta though, since 9.1 beta is just better
My 6s came with 9.0.0.