I wish I could choose between those excellent big lenses. However I don’t think it is an easy choice.
I only have experience with a 500mm F4 P with AF converter and with a 300mm F2.8 AF-S VR with a TC-20EIII, the first giving 800mm and the last one giving 600mm F5.6. The first is fine on a Gitzo series 3 with a gimbal head and the second is handholdable. I find that both can make excellent pictures in the best conditions. The weights of both are a little more than 3 kg.
I would certainly be in doubt on what lens to those. I think it very much depends on what you are going to shoot and how you are going to move around.
I think both the 400mm 2.8, the 500 4 and the excellent lenses but always best to use without TC’s not only because of image quality but also because of speed and reliability of the AF. So if you expect to use the 400mm 2.8 with a TC most of the time, I certainly would recommend the 600mm 4.
The leap in weight from a 70-200 F2.8 to a 400mm F2.8 (with a Gitzo series 3 or 5 and a full size gimbal head) is huge, the 500mm being the lightest.
I suggest that you start to rent one or more of the lenses if at all possible where you live. Not the least will this give you an idea on how much weight you want to carry.
I also would consider what camera to use with the long lenses. I find the AF-function in the D700 and the D300 considerably more reliable on long lenses with TC’s than the D7000. As Nikon don’t seem to produce another pro grade DX camera, I think I’m going to keep my D300 for use with long lenses for a long time.