My advice - go the D800. It looks like a brilliant sensor from every test i've seen. I think the 5dmk3 will give you better video and better high iso, but personally think the new sony-built sensor trumps this.
Despite a recent serious investment in Canon glass, I'm seriously thinking of turning to the dark side and getting a D800E.
P.s. i've shot about 6000 frames so far with my 5dmk3 and i do love it...it's a good, well rounded package. it's just that if you want pixels and dynamic range it's not the best camera for the job right now.
