I had both of the lenses when I had 2 bodies, but after selling the 2 to get a 5D3 I only needed 1 lens. After doing a lot of testing at various lengths and apertures I decided to keep the Sigma and sell the Canon.
They both have very similar IQ and very similar AF speeds. The Sigma has a far better stabilising system (I got down to 1/10sec handheld on the waterfall shot below) and I also found the Sigma was able to focus well for longer as the light was dwindling. The Sigma is sharp wide open (at least mine seems to be) and I was so impressed with the 150-500 I got the 120-400 when the wife got a 650D and I also borrow that at times now I've got another second body.
This was taken at 150mm, ISO100, 1/10sec and f22 (I was trying it out at all apertures to see how it came out). SOOC with no PP at all.
[/url][/IMG]
This was taken at Waddington airshow at 250mm, 1/1000sec, ISO640, +1 EV and f10. This is SOOC with no PP apart from a crop.
[/url][/IMG]
This one was taken on Dec 16th in the back garden, 500mm, ISO200, 1/250sec and wide open at f6.3. Again, this is SOOC with no PP apart from a crop.
[/url][/IMG]