There is not that big difference between a7s and a7s2 exept the internal 4k
Otherwise nothing... and I am saying that from experience.
Downside of both camera's
1. Stop filming after 29 minutes
2. Recording is on 420
3. Chances to overheat.
I have the a7s and Iwant it to trade it again'st a7s2. Wrong...
Instead I got an assassin atomos ouputing 4k from hdmi 422 proress HQ which it means better color to my footage which it also means the camera is on but sort of standby when the atomos recording 422 proress HQ better colors and most of all, no pausing after 29 minutes of internal recording and yet, no overheating issues. Yes, I have found that connecting an atomos it is more work maybe 10 minutes but yet your back to your post with a better footage and no pause issues during filming.
With the saving that I got for the 7s2 I spend it on a7r2 better camera on dynamic range on super 35 4k there on the a7s2 your are limited.
If I need low light camera if it is ctucial I take the a7s with atomos otherwise I take the a7r2 on normal lighting condition recorded on atomos 422 HQ proress the footage is gorgeous... I don' t fell sorry for my choice to the contrairy I feel happy because I made a smart choice...