Gizmodo did a good side by side review of these two new cameras. If course, the Canon is a no-brainer for me because of my existing investment in their lenses. The bottom line:
In the end, for me, I give the trophy to the 5D Mark II for the 1080p video. It’s got its drawbacks, sure, but being able to switch seamlessly from stills to beautiful movies with my awesome 35mm DSLR lens is just too good to be true. The D700 has an edge, albeit a slight one, in some categories like high ISO, but in the new world order, 1080p video from your DSLR is an ace that can’t be beat.
via Review: Canon 5D Mark II vs. Nikon D700 Review Shoot-Out.