If you were buying a camera at the moment, which one of those two would you pick? That is a question that seems to keep coming up the last couple of weeks, and I ended up discussing it again last night with someone trying to make that decision. No conclusions were reached; I think part of the problem is that the head says "The Sony NEX-FS700 [$7,999] is half the price of the Canon C300 [$15,999], it doesn't make sense to get the C300" but the heart says "Yeah, but get theC300!"
As I see it, the pros and cons of the cameras (compared to one another, not to other cameras) are:
- Price (half the price)
- High Speed (Super-slow mo)
- Supports wide range of lenses
- 4K maybe
- No idea what the 4K costs
- Support for Canon EF lenses limited by the lens adapter capabilities
- LCD panel positioning is limited
- AVCHD compression
 Still no info on when, what cost, and what cost for the hardware.
 Some people think AVCHD isn't professional, while others say it's fine. The camera also outputs through 3G-SDI, but the image quality difference between that and the AVCHD image doesn't appear to be noticeable.
- Great low light sensitivity and organic looking "noise"
- Better support for EF lenses
- Better and more flexible LCD placement
- C-Log 
- Dual Compact Flash recording
- Lens choice more limited 
- 8-bit, older compression format 
 You have to choose either the EF or PL mount camera.
 Many people think the 8-bit image is still excellent.