I think I'd likely go with the Sigma 150-600 C. Seems to do a good job and won't break the bank. Your flickr seems to have a lot of wild-life. I would think the 150-600 would give you opportunities you don't have with the 100-400.
I don't know, maybe its unconscious bias, but I think the Canon 100-400 ii is sharper than the Sigma 15-600. I've looked at that lens before thinking about getting it, but from the photos I've seen I think the Sigma is not as sharp. I haven't worked with it on my camera, but that is just my impression from what I've seen and I'm not willing to trade sharpness for extra reach.