I had both the 100-400 mkI and the Sigma 150-600 Contemporary for a while. They are both good lenses but in the end I could only keep one of them. I still have the Canon. Personally I would not give up that 100-400 and the 400 f5.6 for the Sigma. Selling my 400 f5.6 is the biggest lens mistake I've made, I will have another one day.
Never had the 100-400 Canon but have owned the 400f5.6, and I up graded to the Tamron G1 150-600 its always a personal thing with lenses, but I like the extra reach at the top end combined with the versatility of the zoom (makes BIF a lot easier for me being able to zoom out to find the bird them zoom in to get the shot).
Just to add another lens into the mix is the Sigma 60-600 it's a great looking lens and example shots I have seen are very sharpe at the 600 end not as good at the 60 end but gives you a lens that can almost do everything.