The sensor is A+++
At low ISO.
In every other respect the 5D Mk III is equal to, and usually better than, the D800, which is why the Canon is the better all rounder.
The D800 really is a one (or being charitable, maybe two - landscape, maybe studio) trick pony.
not really. the 5DIII could be called the 1 trick pony as well since all it really has over the D800 is a few very high ISO levels which are mostly too noisy for most people's standards and a mere 2 fps ( and just 1fps over 1.2 cropped mode). that is good for some sport shooters (but it is no 1Dx/D4) but not really state of the art for landscape, studio, portrait and wedding which is a huge market and who the D800 is targeting.
ultimately if you shoot landscapes, wedding, studio, portraits the D800 is the better all rounder because:
-offer far more crop flexibility (big deal for wedding portrait)
-offers far better resolution for large prints (huge deal to wedding/portrait/studio/landscape)
-offers far better dynamic range and cleaner shadows and to preserve whites (big for weddings/landscape)
-offers uncompressed hdmi out which has greater editing latitude (great feature for nature videographers with an eye for broadcast quality)
so as usual, it all depends on what you shoot. if you shoot sports, the 5DIII is ok. But if you're serious it is too slow. The way I see it is that the 5DIII is like jack of all trades but master of none. Unless you shoots sports and landscape combined (and the market for that is nearly zero) that tradeoffs of the 5DIII just don't make much sense. Nikon selected a more reasonable approach IMO. instead of making a camera that doesn't excel at any one thing beyond the others ( like the 5DIII), they looked at one market (wedding/portrait/studio/landscape) and gave them the best full frame dslr money can buy with acceptable trade offs to yield benefits available nowhere else.
If canon had done that, I'd still shoot canon because that is what the 5DII was. The 5DIII changed the formula and tried to be a D700 which never did that well.
ultimately the 5DIII tries to please everybody but in doing so, it was held back from being the best at any one thing.