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5d3's live view is faster and more responsive in every way with less blackout time, except the d800's live view focuses much faster.
D800's LCD IMO just doesn't make images very pretty, not like Canon LCDs do.
Focus speed/etc. similar dependent on lenses.
D800 shadows are infinitely better at ISO100, comparable around ISO800, and just lack dynamic range compared to 5d3 shadows past that. 5d3 has less amp glow at top ISOs, so no purple tint at ISO12800/25600 like the D800 has (except extreme bottom right corner on my 5d3).
The actual appearance of noise...D800 noise just looks more organic and less artificial no matter what.
And now I go off to shoot a wedding with 5d3, d800 and a few 5d2s for backup/etc.