I got the A7Rii mainly for 4K video. I love the ability to crop it which I can't really do on the A7s.
Mike, consider this option carefully, it's really handy. Unlike photography, the video quality is about the same in crop and full frame mode, so you can get the benefits of the APS FOV for video. Due to the lower resolution, don't think the a7s2 can do it.