Being a newcomer myself and having been through similar debates/ confusing information before buying my Canon 300 f4 IS maybe I could offer some general tips, rather than technical detail. So here are a few "rules" I developed during my recent experience:
1. None of the lenses you list are "bad". They are all shades of excellence.
2. Owners of a particular model will always say their choice was the best. It's just himan nature!
3. The people who have owned both often provide the most relevant information. But still beware of Rule 2 above.
4. YOU MUST COMPARE THE LENSES PHYSICALLY YOURSELF. For what it is worth, it was only when I handled the Sigma 50-500 and the Canon 300 that I realised how big and heavy the Sigma was (but see Rule 2 above
). Similarly, I personally did not like the pump action zoom on the 100-400 when I held it in my hands (Rule 2 squared).
5. Whicnever you select. you will really enjoy it. You will have a few days of angst over whether it was really the right choice, but then you are likely to become a Rule 2 zealot!!
6. This Forum is a unique way to access the knowledge gained through years of experience from some great people. You would never have had access to this information in the past and might have made choices based on conversations with one or two people. But it does come down to the "balance of the scripture" i.e. you don't take single lines from the Bible, but look at what it says overall about the issue. It's the same with Forums and lenses.
Back to others for the technical bits......
Michael.