To the OP- What is it that you don't like about the 50-500? You say you always shoot at 500, but then you pose the question of two 400mm lenses so maybe I'm missing something. Have you considered the Bigma w-OS? That's the same price point as the 100-400.
I'm especially interested in this thread because I'm at a similar decision point. I'm new to photography as a hobby and I find myself shooting a lot of birds, so I'm looking to buy a nice big lens. I rented the 100-400 a few months ago. Right now,for the next week, I have a Bigma w-OS rental. So far I'm really impressed with the Bigma, but I might rent the 100-400 again before making a decision.
As a tangent, has anyone heard anything about Canon putting out a next generation 100-400? This is a pretty old lens and I'd hate to buy one right before they updated it. TIA!