Let me correct you on some of your statement,
The SIGMA has a LOT more reach!
The "dust pump" effect will happen to any zoom that extends it;s length, via push pull or twist, as the internal volume obviously increases as it extends. People's belif that push pull did something magic to make it worse is inaccurate.
I owned the 100-400mm for over a decade, it was my go to wildlife zoom, and the best for what it did the whole time I owned it.
Now I own the MkII AND the SIGMA C.
The SIGMA Beats the old version of the 100-400mm in every way with two exceptipons;
- Speed of zooming.
But NOTHING beats the old 100-400mm in the latter regard, not even the new MkII, due it's magnificently instantaneous push pull zoom.
For the same price, I would go with the much more "contemporary" lens design, the SIGMA with better IS, better range,. better IQ, and in many ways better AF.