I own both the 100-400 and the mark I 600mm. If the choice was between the 500 and the 600 and you can afford both, then I would choose the 600, as you will always be wanting the extra reach, and since you probably should be using a support for either one, then there is no real difference in usage between the two (IMHO).
Now, I did borrow the 200-400mm and that is an awesome lens and is actually one of the next lenses that I intend to buy. With the built in teleconverter this gives the range of 200-560 and this becomes a highly useful tool. Coupled with a camera that will focus at f8.0, then you can add the 1.4x, and have a great range for the lens. So I would pick this one up first.