- Context: I do not own a Nikon or Canon DSLR. I have had a long string of Minolta & Sony cameras. If Sony do not make their cameras make the sensors that they sell to Nikon work as well as Nikon does at high ISO, and soon, I'll be changing to Nikon or Canon. I value good quality high ISO capability and the ability to take photos at a rate that the situation requires.
See the DXO Mark comparison between the D800 and 5DMkIII that Itai referenced.
Also see the DXO Mark high ISO performance comparison chart for all the cameras that they have tested to date.
The vertical (Y) axis is the maximum ISO at which a specified set of quality measures are met. This is ~~ the max ISO that you can take a photo in normal conditions and expect a "reasonable quality" result. Their sit explains the above in more detail.
As Itai notes, DxO "normalise" their results by scaling to a 12 megapixel image. They scale the signal to noise results down by a factor of square_root(sensor mp / 12 mp). This effectivelu scales the ISO result up by the same amount. So eg a 24 mp A700 would have an increase in the catual rating by a factor of (24/12)^0.5 = sqrt(2) ~= 1.4.
And a 36 mp D800 would have its ISO result "improved" by sqrt(36/12) = sqrt(3) = ~ 1.7.
DxO ISO results for the top 35mm cameras and their megapixel count are as per the list below. IF you believe hat DXO's scaling method provides a good match to reality then you can use the chart below for comparison. This is derived from DxO's table here (same ref as above) and camera mp sizes. While the D800 is shown as ahead of the 5DMkIII by 2853/2293 ~= 1.24:1 that's about 0.3 of a stop. Notice it, even by pixel peeping, try as you may, won't you. The now venerable D700 sits happily between the two. Remember that his is when the image has ben adjusted to 12 mp by whatever magic - so the awesome 36 mp of the D800, if you had set everything jus so o achieve it, has been obliterated. The 4 year old D700 is still right amongst them. The D3s is just noticeably above the D800.
The second chart has had the ISOs adjusted back to what is actually measured before allowance is made for sensor size. I divided the reported ISO ratings by sqrt(12/mp).
Results are sorted in order of raw ISO.
D3s is top. Again. No surprise.
D4 is second. You'd hope so.
D700 & D3 (same sensor in fact) trail just noticeably behind.
A small cliff before you get to the 5DMkIII, D800 (gap so small as to not be noticeable) , D3X, 5FMkII and a slow ail down to the D5100 & D7000. The should be ashamed of itself Alpha 900 shares bottom place with the best of the APSCs.
While their is a mini cliff between per pixel D3/D700 amd D800/5DMkIII of about 2300/1680 =~ 1.4:1 this is still only about half a stop!.
Conclusion: If you want good low ISO resuls and you believe the tests and the method of scaling gives a realistic basis for comparison then there is only half a stop across the range D3 - 5DMkIII - D700 - D800 - D4 - D3s in that order. D3s is still king.
If my eye-brain system is to be believed (and why should it be ;-) ) there is somewhat more than that between them.
Re Tom's D7000:
It is about 1 stop down on adjusted ISO wrt D800 or 5DMkIII
and somewhat more than half a stop on raw ISO. Not vast in either case.
I'd strongly suspect you'd see a greater difference that that in real life - based on my peering at comparative high ISO images for other cameras.
However - if you want real world performance and are interested in buffer size and peak shooting rate I'd look very closely at comparison tests in environments of relevance to you. I've seen comparisons of the two cameras in situations where the nominally superior Nikon performed very badly indeed in tests that were relevant to me - not on noise, where it was comparable to the Canon, but in usability to take actual shots in real world situations -
frame rate in short bursts
effects of buffer size and file size and data transfer rate, and
In at least the first two the D800 is behind.
As much as I hoped it was going to be a D700 successor, it's not.
Vanishingly few people would have chosen the D700 for studio shots.
When comparing it to the D800 or 5DMkIII it's in extreme lower light situations and/or where there's plenty of action needing bursts of rapid shooting and possibly as close to semicontinuous shooting on occasion.
My personal conclusion is that the D700 is (still) looking good (4 years on), a D3s would be very nice, and I can hope that Canon has got its act together with the yet to be judged 1DX.