David,
I use a A7II as my travel/documentary camera, and find the image stabilization very valuable. Especially if your lenses do not have image stabilization, I would go with the A7II.
The low light capabilities of the A7S come at the expense of having only 12 mpx, if you can live with that, and use image stabilized lenses, perhaps it will be appropriate for you. But the A7II also delivers very good results at ISO 6400, are you going higher than that?