The images I've seen so far from samples and reviews online of the 5D Mark III zoomed in past 100% to the D800 at 100% look as good or better when ISO noise and tonality is considered. But this is so minute it's not worth considering, there is no real world advantage that I can see.
If you want to see a real world advantage to having a higher resolution image, check this out: http://www.phaseone.com/en/Downloads/Select-Download-Server.aspx?Item=722c6f07-891b-4edb-b307-54bb3d2f7bfd&Language=2776733f-13f6-4887-ba49-d4a9168a79d0&RDR=0
Never mind, there old sample was easier to open, this thing is a mess of weird RAW files I know nothing about, the older style sample was a massive 120MB .tif file that looked utterly stunning, this must be samples for people who are familiar with Phase One file formats, not for casual onlookers.
5D has awesome ISO from what I've seen in samples (appears at least very easily arguablly a bit better than D800 but I don't think it will lead to any advantage in most situations). The camera shoots faster and doesn't suffer a buffer problem after shooting a large burst, so that is a definite real world advantage. It doesn't have a pop up flash, I actually wish it did, that can be useful, the D800 does. There's more to consider (video/etc), but honestly, I'd be very happy with either camera but would prefer the FPS of the 5DIII and forgo what I consider an on paper only resolution advantage and the lack of a pop-up flash. Also, the one video test I saw of the D800's clean HDMI out vs.the same D800's internal recording showed absolutely no difference in terms of quality between the two (at least on the highest res it was posted at on Vimeo), but that was just one sample, both cameras suffered a small bit moire issues on certain textures/patterns in the picture but each on in slightly different spots of the same image, lol (maybe this is because they were filmed side by side and the lighting/angle was ever so slightly different) They looked about the same to, not absolutely amazing, but easily usable for professional work in the right hands. For video, without knowing how they feel and the control styles, I would be equally happy with either from the samples I've seen so far but am not 100% happy with the quality of either.
My gut is telling me, after the dust settles and the rushed hype settles, Canon actually has the winner here but it is down to preference and Nikon users have nothing to really desire for other than faster FPS. But this debate will not die until probably even well after the 5DMKIV and D900 launch.