This is not going to be a long-drawn-out review with charts and samples, I'm not going to show you the different menus and discuss how I think Sony should have done this or that.
I'm just going to give you the same conclusion, everyone else on the internet that has had the chance to review these two cameras, will give you. If you are shooting video get the a7S II - if you are going to shoot mostly stills, get the a7R II.
The a7S II is much better in low light, you can practically shoot in the dark, really. It makes great 4K video with 1:1 pixels and no bining. Whatever that means.
I shoot mostly video, but I decided to keep the a7R II because it shoots incredible stills and the video is great, besides I usually light what I shoot. The a7R II is pretty good for low light as well - not as good as the a7S II but it will do.