I have both as well as the a7r. The a7s and a7ii purchases were for difference specific purposes. The a7s is a specific tool that can't be replaced by any other camera at this point. For High iso as everyone pointed out it is unparalleled, but is also has not only the electronic first curtain but the silent mode. That silent mode is what will allow be to use it as behind the scenes production camera where I can shoot behind a principal photographer with absolutely no sound being made. Before this camera I would use the rx1 with its leaf shutter that is nearly silent. This takes it to a whole new level being completely silent. So for street and pure silence its a great tool, but the low megapixel count isn't going make it a primary body.
For me the a7ii is closer to that goal, I haven't shot a ton with it yet, but the ibis seems nice and the camera seems quite capable. I would use it for a much broader range then the a7s, but in those situations where the a7s shines, it will be my go to tool.
For instance,
I shot this from my seat on a recent United flight coming into Chicago. This would just not be possible with any other camera as I'm shooting through gold tinted airline glass at night with clouds in a fast moving jet. I only get one chance to make these shots while travelling so I want to have the best odds possible. In this case I could barely see anything from the cloud cover until this chance opened up to shoot the skyline.
Shot with the Sony a7s at 256k ISO
Through the Clouds by
PeteTsai, on Flickr