I'm shooting my movie, now with 2 5D3s. I've got Canon lenses, and I am familiar with Canon's software (custom profiles), menu systems, and can get pro audio out of in-camera recording when necessary (using a preamp).
I can sharpen and add custom noise to IPB in post that looks crisp, clean and detailed enough for my needs at this time.
If the D800 were all around better, would you sell all of your Canon glasses and switch?
The FS700 is more than twice as much as the 5D3, requires an expensive adapter for Canon lenses (Metabones), requires a full support rig and a lot more gear to make into a useful tool. Granted the results will be excellent, however run & gun and discrete shooting options go away.
Hopefully the release of the FS700 will bring down the price of the C300, which I would prefer over the FS700 (the FS700 won't do 4k at release).
For the manufacturers, yes, it is a contest of sorts. However for us consumers, the release of new cameras doesn't prevent our current cameras from working. Nothing stopping us from completing our projects.
Also note that a $450 maxmax.com OLPF removal mod may bring the 5D3 up closer to 1000 lines of resolution. If they can make the dust cleaning system operational, I might consider this option (after more examples have been posted showing no major issues with aliasing and moire (at least similar to C300 performance)).