Hello my friend...
Firstly, both cameras are unreleased and not properly tested so there's lots of conjecture.
D800 is 25mbit/sec, 5D MKIII is 91mbit/sec, so for in camera compression the 5D is clearly ahead. The question then is how good is the Nikon's HDMI out? It's an unknown quantity currently. Do you want to be faffing with an external recorder on the Nikon if the 5D's 91mbit is good? I think if you expose and light the 5D right in the first place you won't need the extra bits so badly because you won't need to do much grading in the first place. Unless of course you plan on using it for greenscreen, but i'm sure you'll be using a better camera for that anyway. If the 5D MKII was good enough for House it's not really down to the bits so much...
5DMKIII will be better in low light. There's a test with good and perfectly useable results at 12800 ISO!
5D appears to have better resolution for video.
Dynamic range i don't know. Nikon seems better to me from some stuff i've seen but this could be moot since there aren't any real tests done, and the 5D stuff was probably shot badly.
Aliasing/Moire. 5D MKIII appears to have pretty much eradicated Aliasing/Moire as you can see here...
With the Nikon however it's still there. Also, rolling shutter appears worse on the Nikon.
Colours - as per your preference. Question is whether the HDMI is going to give you better colour separation and depth than the 5D's 91mbit, currently unknown.
Stills, both will be fantastic i'm sure.
Ultimately, getting the most out of the cameras will be down to good glass, lighting, and setting them up right. But as to your question, you'll have definitive answers in a month or so when everyone and his dog will be posting minutae peeping side by side comparisons.
Hope all's well
K