It's quite spooky as I have a similar dilemma.
I already own the 100-400 (Mk1) and the 150-600C. I bought the Sigma as, despite the Canon being a lovely lens, it often left me short when birding, especially on a FF camera. I bought the Sigma for the extra reach although sometimes, even that is short. The Sigma though is as good as, if not better than the Canon, it's certainly as sharp, if not sharper.
I have been considering updating my 100-400 to the Mk2 but I'm questioning if I need to, given how pleased I am with the Sigma. The only drawback with the Sigma for me is the f6.3 at 600mm, but to get any 'faster' would cost big bucks, so the Sigma will have to do.
As for reach, 600 (960 equiv on APSC) is great but even that, sometimes isn't enough, it's upto me then to get closer.