DxOMark.com, which analyzes DSLRs, sensors and lenses has given the sensor in the new Nikon D800 (see the full report here) their top ranked camera sensor, with a score higher than any preceding camera, including top medium format backs. And the D800 is $3,000 USD ($500 less than the 5D Mk III).
For contrast the DxOMark.com rating of the 5D Mk III is here. The 5D Mk III sensor rating is only slightly (3 DxOMark points) better than the 5D Mk II whereas the D800 is rated 15 DxOMark points better than the D700. Looks like Nikon may have hit it out of the park with this one!
Of coarse sensor performance is not everything. Handling, video features, etc are all important but those reviewing the D800 and the 5D Mk III (see here, here and here) seem to be very positive about the D800.
As Michael Riechmann says in his review, "Experienced photographers know that quality lenses trump everything else in the image quality chain. A good lens can make the most of a sub-par sensor, but a poor lens on a great sensor is a waste of money. To coin a phrase – a chain is only as strong as its weakest link, and this is especially true in the camera/lens equation". So if you are going to get a D800 for real estate you'd better also spring for the Nikon 14-24mm f/2.8 G ED wide angle lens, if you don't already have one, because you'll need top notch glass to get the best out of this top sensor!
But wait, Ken Rockwell says "screw Nikon", he likes his 5DMk III better than his D800 because of the handling. Who can you trust on these things?