Well having owned a 50-500 and now owning a 70-200L IS
and 100-400L IS
i think i can comment pretty fairly.
The Sigma 50-500 is a truly awesome lens in every respect.
Its sharpness at all focal lengths over a massive range is pretty unmatched by any lens made by anyone at the moment.
I sold mine as i was finding my wastage rate was to high caused mainly by shake at the longer end and the lighting conditions i was experimenting in.
This is better now with the 100-400L but the difference in quality is pretty slim and i am missing the 400-500 bracket which comes in handy in Sydney.
I really wouldn't be comparing it to the 70-200 as it cannot compete with the Sigma for sharpness with a 2x TC
attached outside 200mm and you are still short 100mm worth of zoom even with the TC
I am considering buying another to get back the 500mm reach its that good for shooting in good light.
However technique,wind etc play a big part in your shots at those lengths and lots of practice is required.Using my 10D its effectively a 785mm lens and sharp as to boot.
The Sigma 50-500 is an awesome lens for its price,hey its simply awesome.
Many shots on this site will attest and it will give you a lot more flexibility than the 70-200 assuming you do NEED the length.
If you don't need the 200mm+ range i'd go with the Canon but if its long range sharpness your after the 50-500 is exceptional value and then some.