I haven't read the whole thread, but here's my reply to the OP.
In its latest issue, the French magazine "Chasseur d'Images" did a side by side review of both the 5DIII & D800. There conclusion is not at all that of the OP.
For those who don't know, Chasseur d'Images is a respected review, that has very systematic testing protocols. you might not agree with there approach, but you can't deny that they come closest to what you might call scientifically grounded tests.
The review of the two cameras where both in the lab and in the field. TO make a 30 page long story short, here's their summarized conclusion.
The resolution advantage of the Nikon is only relative as it will matter only for prints A2 (40cm x 60cm) and higher. However, Nikon is a clear DR winner with 14 il at 100iso. The Canon only has 12il. The difference equalizes as you move up the iso scale. They do note however that the Canon raws still give plenty of room for highlight and shadow recovery, and that they can sustain severe post treatment. At higher iso, the DIII pulls ahead (less noise), delivering useable results at 12800, where the Nikon pretty much breaks down. Still, they rate the high iso of the Nikon impressive given its resolution. At 3200iso, the 2 cameras are practically equal.
AF on fast incoming mobiles yields a slight advantage to the Canon. All in all, however, both cameras seem to deliver very high, responsive and reliable performance in this department.
In spite of its less sophisticated metering system, the Canon delivers very good and reliable results. So does the Nikon, but that should have been expected as the D700 already excelled at that.
Noise : the Canon has a clear advantage over the Nikon, even in standard mode. The canon silent mode receives raving comments : it makes it possible to use continuous shooting in a concert hall. The camera might be heard only during a triple pianissimo or a flute solo, the reveiwer says. Sound measurement in silent mode yields 52db, more less the same level as the mirrorless Sony nex5.
menus and interface : there seems to be an agreement that the Canon is ahead.
Viewfinder : both are bright, crisp, and pleasant for people who wear glasses.
Very few negative points on either cameras. Nikon : the size of the files which makes it imperative to have up to date computer equipment. Canon : the 1/200 sync speed which receives a real bashing. Oh, and Nikon's program mode in video seems a bit inferior to the Canon, but I don't do video so I didn't fully understand : it seems D800 allows excessive shutter speed (?) .
My conclusion from the full reading of these reviews : both cameras probably exceed the needs of most photographers. What really differenciate them is 1. Nikon's extra 2il of DR at low iso ; 2. Canon's silent mode, and ultra high iso (12800 and up).
In other words, people shooting events, shows, concerts etc, might be better off with the 5DIII, while landscape photographers should opt for the D800. Still the reviewer concludes that both cameras are impressively versatile, so there might not be very convincing reasons to switch if you have money already invested in one system.
Hope that helps.
... and 3rd party lenses fit on Nikon mounts, too. But seeing the latest Canon price tags (doubling the price for "upgraded" gear) I have to somewhat agree with clicstudio. The long-run full frame upgrade path in the Canon world is very expensive, while other brands might be quicker to make larger sensors or more mp available to the general public.
... I don't know Nikon rumors, but it would be interesting what the reason for this reason in culture is (if there is indeed any).
