Hi
I have been fortunate enough to own both of these lenses and used them both a lot. I sold the 500mm F4 to get a the 150-600 Sport ( a story for another day). I kept the Sigma for less than a year then sold it and bought a 500 F4 Mk2.
IMO there's no comparison between the 2 lenses. I found the AF inconsistent on the Sigma and always felt like it delivered 1 shot in focus,next out,next in etc. The lens delivered sharp enough results but forget about adding a tcon to it for anything other than static subjects. The 500mm on the other hand is excellent and also works brilliant with the 1.4 Mk3 tcon attached. They are both heavy buggers but you felt like it was expected with the 500mm F4. The Sigma on the other hand always felt like it was too heavy for what it was.
One thing to think about is that I think Canon no longer officially support the 500mm F4 mk1. That's not saying you can't get it fixed if it goes wrong but something to consider.
I've also used the 400 F5.6 and 1.4x tcon and found that a good combo that always delivered nice results. On par with that is the 100-400 Mk2 + 1.4Tcon with the added benefit of the IS and a super sharp bare lens. I actually preferred the 100-400 Mk2 + Tcon over the 150-600 and just put up with the extra loss of light due to the tcon.
Also worth looking at is the new Sigma 60-600 which looks like it has been designed to give the best results at 600mm.
Should add that the above is based on use with a 5DMK4,5DMK3,1DMK4 and 7DMK2.
Gary
