See, the thing is, there really are damned few real-world scenarios where the image quality of the 5DIII is inadequate but the D800 is. If the 5DIII won't cut the mustard, the upgrade you need is to medium format, not a higher density 135 format.
I'm not saying that more image quality would be unwelcome -- far from it. But the only times you'll be able to tell the difference between the 5DIII and the D800 in studio or landscape photography is in stick-your-nose-to-the-paper side-by-side comparisons of 32" x 48" prints. Stand back at normal viewing distances, put the prints on opposite walls, or simply print at something a few inches smaller, and you're really not going to tell the difference. And an ill-timed sneeze during exposure could easily tip the scales the other way, as will any other cock-ups in technique.
The sorts of hardware-related things that will make a difference? First of all, the glass, and Canon's got Nikon beat hands down. Nikon has no equivalent of the 17 and 24 TS-E lenses, they have no equivalent of the 65 MP-E, and their supertelephotos weigh half again as much as Canon's. Then, there's all the camera-type things, like framerate and autofocus and what-not, and the 5DIII again has the edge here, but not by as much.
So, yeah. If you're already shooting Nikon, you should be thrilled with the D800. It's a marvelous camera, by all accounts I've read. But all this wailing and gnashing of teeth over which body is best is quite silly, really...the body is really just an accessory, when it comes right down to it. It's like people fighting over Ford versus Toyota based on the shape of the cupholders in the latest midsized sedans. I mean, really, people?