I was debating buying the Sigma150-500 or Canon but was talked out of them both by someone in Jessops and told to get a Tamron 200-500. Not an answer but a third choice, I have yet to try the Tamron but both the others both seem excellent when tried out in a shop.
I use a Tamron 17-270 at present, in fact that is all I ever use and it is excellent, light, fast, and can take some good shots in the right hands so would not hesitate getting another