My Tuppence worth. As a Nikon user I don't know about the Canon lens but I would certainly go for a Tamron 150-600 rather than a Sigma 150-500mm. When it comes to sharpness and clarity, the Sigma doesn't come close to the Tamron.
I would hope that the Canon lens is at least as good as the Tamron.
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Unless Sigma have a new 150-500mm that I haven't heard about, this is an old lens which may have been OK when it was released around 10 years ago but was massively superseded by the Tamron 150-600mm and the Sigma 150-600mm contemporary lens - which both retail at around 15K. The Sigma 150-600mm Sport is in another price bracket at around 30K.
If you can get either of the cheaper lenses second hand, go for it : they should go for around 10K.
As for the old Sigma 150-500mm; I wouldn't go anywhere near it, even though you can get new ones for less than 10K, this is old stock that they are desperate to get rid of. It is nowhere near as good as the Tamron 150-600mm or the Sigma 150-600mm Contemporary (which I believe is of similar performance to the Tamron).
If you want to wait, Tamron have just announced a Mk 2 150-600mm lens which is said to be a rival to the outstanding Nikon 200-500mm but is not as expensive (which remains to be seen). I traded my Tamron 150-600mm in for the Nikon 200-500mm a few weeks ago and I'm staggered by the images I'm getting, so if the new Tamron is even close to that, I'd have one of those.
To sum up my ramblings above....
The Sigma 150-500mm is not a very good lens by today's standards.
The Tamron 150-600mm Mk1 and the Sigma 150-600mm Contemporary are very close in performance and price and are much better than the one above.
The Sigma 150-600mm Sport is a better lens than all of the above but is quite pricy.
The new Tamron 150-600mm is said to be better than all of the above but the price in SA has yet to be announced.
I've no idea where the Canon 100-400 rates when compared to these lenses.
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According to Senscore (LenScore) the Canon EF 100-400mm f/4.5-5.6L IS II USM rates better than the Sigma 150-600mm Sport, both of which retail at about 30K in SA. But the Sigma gives an extra 50% reach!
In the end, it's what you can afford. Just don't buy the Sigma 150-500mm!
Hunting cannot be considered a sport as all contestants in a sport should know they are playing the game!