I put together the weather-sealing diagrams from Canon and Nikon on these cameras and it looks like the 5D3 is more robustly sealed. The 5D3 is also quite a bit better then the 5D2 it replaces with a larger use of gaskets around many of the dials. Canon seems to have done well sealing this body for the elements. Both bodies look great though in general.
I notice the 5D3 has additional sealing in the top LCD, the mounting brackets for the camera strap, tripod mount, diopter adjustment wheel and two switches on the back of the D800 which lack any sealing (from these diagrams). Not sure of the function of these switches so I don't know if sealing would be required, but I am guessing they would be. But who knows
Just a little side by side comparison:
And the 5D2:
I notice the 5D3 has additional sealing in the top LCD, the mounting brackets for the camera strap, tripod mount, diopter adjustment wheel and two switches on the back of the D800 which lack any sealing (from these diagrams). Not sure of the function of these switches so I don't know if sealing would be required, but I am guessing they would be. But who knows
Just a little side by side comparison:
And the 5D2: