I would never choose the 32GB version. I go (insert word here) out and get the maximum possible. Even 64GB was too small. I mean I came over from an Android phone that was dual SIM and had the microSD card slot. Getting used to not having all that was quick and easy. To replace my 200GB microSD card, I bought storage on iCloud equal to that amount and I have a Sandisk WiFi stick with that amount.
I have a 128GB version of the 6S Plus, and believe me, could not have been happier.
One other thing with doing that, is that you won't run into space issues. Archiving photos off the camera roll (separate process within iCloud storage if you have the space), is super easy, now that you can upload multiple pics.
As for space, I would not say 10%. I would say 15-20% depending on how much empty free space you have. Empty in this case refers to the iPhone with nothing on it. Before you load apps.