The 100-400 is a good lens, albeit very dated now. It has the known dust sucking feature, and eventually, whatever is inside the push/pull, along with the locking ring, will give up the ghost. You will end up with little black pieces that fall apart and out of the lens. Happens on a lot of them.... IS is old too, maybe 2 stops if you are lucky.
The Sigma 50-500 is sharper at 500mm wide open than the Canon at 400mm, but the Sigma at 400mm is a tiny bit less sharp. It has other features though like macro mode at 200mm or so, much better IS, and a wider range.
I have tested 4 100-400Ls against the Sigma, and my original 100-400L that I had before getting the 50-500 was the only one that beat out the Sigma, in all other 3 cases, the Sigma was the winner for IQ. I really wanted to go back to the slightly smaller lighter 100-400L for transport reasons, but couldn't ever do it. Now I have a Pelican case, and the Sigma fits there nicely, transport issue solved!
Pics from the 50-500 on the 7D from this pic onwards, and many others in the gallery too.
One of several sporting shots at ISO 12800, which you need if you shoot indoor sports with this lens:
That all being said, I had to send my 50 1.4 into Sigma along with a host of test shots showing the issue. Even +20 on a body that had MFA couldn't bring it back to life. They calibrated it and I am now very happy with its performance.