At 400mm, there is really nothing out there that can touch the 100-400mmL or the 400mm f/5.6L in the under $2,000 range.
I have owned the 100-400, 400 5.6, 300mmL, and a host of others. I ended up keeping the 100-400L as being very good, and easy to carry in a bag because its short when telescoped in. It also has good resale value, perhaps better than the other two.
I bought a D800 recently and am selling it. There is no good low cost telephoto solution that comes close to my 100-400mmL. The best of a bad lot may be the Bigma 50-500 OS, but I've seen few objective tests. The NON OS version was not all that good.