I have owned both the Canon 100-400 and I now have the Sigma which is in the classifieds.
They both have their pros and cons.
Canon
Pros: Better construction and mine was a sharp copy and I got some real nice photos right up 400mm
Cons: The trombone zoom action is not to every ones taste and this mechanism does attract dust into the lens body. Certainly would be more expensive than the Sigma.
Sigma
Pros: longer reach (that extra 100mm does make a difference) better IS than the Canon and sharp up to 450mm. Would be cheaper than the Canon
second hand
Cons: Above 450mm you need to up the aperture to f8 to get the best of the lens.
Both
Pros: After some practice it is possible to get really good images with both hand held even at maximum zoom
Cons: Both are heavy and to get really great images both benefit from a stable platform (Tripod, monopod, wall or similar). Both have been replaced by newer versions.
In case you are wondering. I gave my Canon to my son who still uses it occasionally. I now have no need of the Sigma for two reasons. Mainly I no longer take photos which need that sort of lens and also due to an arm injury I cannot handhold my camera and the lens. I really cannot be bothered with carrying a tripod around.
Whatever decision you make I am sure you will get a lot of pleasure playing with the chosen lens and learning the required techniques to get the best results.
Good luck