For video, it is known the D800 is sharper out of camera... However, the 5D3 takes better to post processing. More detail comes out of sharpening the softer 5D3 video then having it be sharper out of camera. That is what Chuck the Canon tech guy has said, at least.
That's also what 'The Camera Store - Nikon D800 vs. 5D Mark III Part 3' and Philip Bloom said.