The main differentiation is cost per performance.I've been trying to keep an open mind about these, but I'll almost certainly have a 100-500 and a 1.4x which gives me 700 mm at f/10 in a lens which is just a few mm longer than the 600/11 DO when retracted and much shorter than the 800. I also get (with the 1.4x) around three times the maximum magnification for butterflies, snakes etc., and I won't need to buy any new filters! Or indeed a lens hood. The only disadvantage I will have is weight but even that is not a huge difference vs the 800.
The 100-500 will probably cost 3000$ and 500$ for the 1.4x extender (at least).
It's sharpest point won't be at 500mm and the extended may degrade that slightly as well, I don't see it being as good as the 800/11 and it may have a somewhat busier background rendering.
But we are comparing different things altogether, a long prime is always going to be much more specialised, and with a slow aperture, that takes it even further.
Actually, that's not necessarily a bad thing, as it basically forces its user to maximise its narrow operating range.
(Although I am sure most people would curse if something suddenly came closer to them, and they couldn't achieve focus or if there is not enough light to get a usable image)
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