I'm filming tomorrow too, likewise first time with this camera.
I will be going with Philip Bloom's advice to use Cine Gamma 4, given in the comments following his A7s review. http://philipbloom.net/2014/07/03/a7s/
Some extracts follow:
Philip Bloom: ...just don't shoot in S-Log Eduardo...I don't Cine Gamma 4 is very flat indeed and you can be at 100 ISO! July 5, 2014 at 8:21pm
Philip Bloom: ...don't use s-log use cine gamma 4. incredibly flat. barely any difference and you can shoot at iso 100 · July 5, 2014 at 12:23pm