The problem Canon has with pricing and sales will not be the Mark III. Most willing to pay $3000 will figure out a way of getting $3500. But for $700 more than the Mark II, the Nikon D800 is tempting. Personally I prefer the Mark III to the D800 (I prefer to have low light abilities and speed than MP), but the D800 is a more modest upgrade from current 5D mark II owners and offers improvements everywhere if you don't mind switching.
Depends on what you are considering upgrade...
In terms of cost, Nikon is a smaller jump, and yeah, you get a bunch more pixels. In terms up upgrade, IQ and ISO on the Canon look very very good from the early shots, and ISO 25,600 versus 6400???
I think the Nikon might have a little bit better metering, but in terms of AF and ISO, these look stellar for Canon, especially if low light is necessary... like a wedding photog needing to shoot in a church.
I think in the end, the Nikon is an easier "step" for some to make, but the Canon is the "bigger" step in terms of quality. Canon also appears to be faster FPS as well. Not surprising given the MP difference, but I think side by side, Nikon gets Metering, MP, and maybe some small video, but Canon kills it in ISO, IQ and AF seems like it may be a win as well