I have a D3 and a D800, and used to shoot with work's D700.
For a similar price, I would pick up the D3 over the D700 because it a much more responsive camera. I bought the D800 because it does something which the D3 can't do — oversized files for easier advertising shooting — while retaining the ISO performance.
Once you've kitted out the D800 with the battery grip _and_ the D4 style battery with its super-expensive charger, though, the price has risen by almost another 50%.
The D4, by contrast, was a camera I absolutely loved, but it didn't give enough more than the D3 where the D3 was strong, especially not for the price.