It´s true that the 5D Mark III is only better in speed and AF.
But I don´t like the image quality from the D800E. In my opinion it´s not advisable to put more than 30 megapixels on a FF chip.
If you compare the image quality from the D800E with the image quality of a medium format camera the D800E loose, because the image is totally unsharp. But it´s all a question what are you willing to pay for the best image quality.
in what way are the d800 files lacking?
in all my tests the d800 absolutely leaves my 5dmk3 files for dead. far better dynamic range, especially in the darks. when the darks are lifted, the file just lightens, doesn't have any of the ugly clumping canon grain.
I have only tested up to 1000 iso (the most i ever need) , and the d800 looks better then as well- more film like in the grain. The 5dmk3 has a terrible low light quality- especially when the light has a colour cast.
the resolution is a lot more, upsizes better.
if you test the cameras- you will see that more than 30mp does hold up with sensor. the files are sharper, the graduations are smoother. have you actually shots both cameras side by side?
i also have a 60mp phase back on a contax, and although this is sharper than the nikon, i think i prefer the d800 files for the look over the phase (that has the same horrible low dark grain as the canon).
also, the tethering into a macbookpro retina with usb3 is a lot faster than the 5dmk3/usb2 - even though the files are almost twice the size.
I have tested a rented d800 for 2 days now side by side with my mk3, and file wise i can honestly sat the nikon eats the canon.
i dont really like the handling or the lenses of the nikon, so im holding out for canon to come up with an answer to the d800.