I was going back-and-forth between between the GH4 and the XC10. I ended up going with the XC10, but returned it a week later.
I really wanted a camera that was built for video, had great ergonomics, could shoot slow motion, and had 422 color without needing an external recorder... The XC10 seemed to tick all these boxes. But working with it was something different. There is a single internal ND filter, and it's not strong enough for shooting outdoors. The headphone jack is IN the hand grip. The lens is a bit of a problem. I thought I'd be OK with a parfocal but varying aperture lens, but the more I used it the more frustrated I became.
In the end I returned it and spent the extra money to get the C100 Mk2. I feel bad for Canon, because if they had improved the lens, added an EVF with a decent eye cup, and fixed a few small design issues, then they'd have an exciting product.
Comparing specs, it looks like the RX10 ii outperforms the Canon XC10 in almost all specs. However, the XC10 write 305 Mbps for 4K while Sony writes 100 Mbps. I can't imagine Sony's compression is that much better, so Canon might have better color?
I'm also considering a C100 mk2 (I still shoot on a 5d mk2). What really attracts me are the ergonomics, scopes and autofocus capabilities. I'm a solo shooter and using a DSLR + Rig + Viewfinder + external monitor + etc. gets exhausting after awhile.
What I'm curious about is this: Can you shoot handheld with the GH4/A7s if you have an IS lens? (I'm not steady enough without IS) Or do you need a rig? How much extra gear do you have to pile on the GH4/A7s to make it a video camera for a solo shooter?