After having used the Sigma 150-600 C for some time now, I think I can confidently conclude that:
1) I have no regrets selling my previous 100-400L for the Sigma.
2) If money's no issue, I'd like to get the 100-400L II, as well. These two lenses aren't apple to apple comparison, really.
3) 100-400L is superior for air shows and probably BIF and other action shots than the Sigma.
4) The Sigma at 600mm and 100-400L (original) at 400mm are about the same sharpness-wise, perhaps even a slight edge to the Sigma.
5) When the subject is very far, however, I believe 100-400L does a better job given the same atmospheric influence.
6) I can rely on 100-400L at pretty much all circumstances; with the Sigma, I need to be lot more selective.