I own both the A7S and the A77II (which just received XAVC-S in the 2.0 firmware update last week) so I shot a quick comparison today. Download the master in ProRes LT to really pixel peep.
To make the playing field as level as possible I used the same A-Mount lens on both cameras and also set the A7S to APS-C mode, which I use quite frequently in the real world. Picture profiles and other details are listed at the end of the video.
I didn't have a lot of spare time today, so excuse the less-than-awesome slider shots. Shouldn't detract from the results.