This is a question for shooters who are used to shoot sport films:
If you had to make a choice between the GH4 or Sony a7s cam to shoot a film about motor sports, which way would you go? and why?
What would you pack with it for a week end shoot outdoor?
It's notable how much the a7s suffers from rolling shutter as opposed to the GH4 and considering you'll be filming sports, quick pans might be something you do regularly. Based on this assumption alone I would recommend the GH4. There could be other factors though!
The one thing all sports shooters want is good slow motion.... Quick pans are less important in my experience. Neither camera has great slow motion, but the GH4 has the edge. It has 1080p96 with decent image quality. The 1080p60 coming out of the A7s suffers from low resolution and aliasing.