maitani
Well-known
the D800 is about resolution, although the iso performance is impressive for the resolution, it was impressive on the D700 too, they seem to be about equal (as mentionned earlier by somebody the D800 gives you the advantage of extreme cropping basically lossless.. and after re-sizing and processing, files at 12 should look slightly better, but personally I avoid heavy PP so the output of the D700 is perfectly fine for me) if you're after ultimate ISO performance, go for a used D3S, or the D4 
also those viewfinders are great and from my feeling esp. the D3S is around 1.5 stops better than the D700 this might make the differenc for you. personally I rather rent if I need the resolution, or the absolute iso performance for a specific task, as the output of my D700 still impresses me, it's still a great allrounder. I'd think about a (used) D800 when the D700 makes its last 'click'
also those viewfinders are great and from my feeling esp. the D3S is around 1.5 stops better than the D700 this might make the differenc for you. personally I rather rent if I need the resolution, or the absolute iso performance for a specific task, as the output of my D700 still impresses me, it's still a great allrounder. I'd think about a (used) D800 when the D700 makes its last 'click'