In the Spencer Boerup review, he claims that the 5D3's ISO 6400 is better than ISO 800 on the Mark II
But on DP Review's RAW file testing in a controlled studio, the 5D Mark III actually looks WORSE than the 5D Mark II
Who to believe?
http://www.dpreview.com/previews/canoneos5dmarkiii/12
Neither. 
There's about a 1/2 stop difference in ISO noise, perhaps more at ISO25600 because the 5D2 really breaks down at ISO25600. I wouldn't use 25600 on either cameras for what I shoot anyways, however in an absolutely emergency, I would actually use ISO25600 on the 5D3. I would never think about using it on the 5D2 however, so I suppose that's a real improvement. (?)
As for IQ, I haven't noticed anything glaring between the 5D2 and 5D3.
IMHO, the real star of the 5D3 is the AF system.



