Canon 5d mark III is by far the more versatile camera. I canceled my order however since it is at least 500 euro overpriced and it does not really add something new for landscapers. If you shoot weddings or even sports and that kind of stuff it is better then nikon d800 by far.
The canon 5D3 has its place for certain uses - low-light events being one of them. At 36MP, I'd never use a D800 for a wedding ... that's just way to much data to process for an image heavy shoot. However, given the specs of the rumored 3D, I feel like I'm being screwed by Canon. At this point I'm seriously debating selling the 5D3, dropping back to a 5D2, and waiting for the 3D.
Given the improvements rumored for the 3D, the 5D3 doesn't seem to hold much market value with the 5D2 still on the shelves. My perfect lineup would be a 7D, a 5D2 and the 3D (with the current rumored spec sheet). The 5D3 falls by the wayside in my mind - but that's just my $0.02.
I've never owned a 5D2 until last week when I found a used one for $1300. I can tell you, that my 5D3, BLOWS away my 1DMIV and the 5D2 by a country mile. So for everyone that can't tell the picture quality from the 5D2 to 5D3 needs to get your glasses check or you are just kidding yourself.
The new AF system & SOOC are shoots at the HIGH ISO are just blow away the 1DIV.
If everyone is putting so much weight on the 5D2 and how wonderful it is just PROVES that the 5D2 is still relavant and way before it's time for technology that is almost 5 years old, but for now, my new/used 5D2 will sit in my bag as a backup, just because it can.