Rodeo, we own the Sigma, it is the earlier version and it works very well on an APSC body with or without a 1.4x extender. That is the good part.
The negatives.
It is heavy, as a 100-300 it weighs more than the Canon 100-400 so adding the extender only adds more weight to get it to the same focal length as the Canon is bare.
If you want to use a filter on it you are in for more dollars, the 82 mm filters are expensive.
It doesn't have image stabilisation, the Canon does.
The Sigma is about $400.00 less than the Canon (going by a quick look at a popular grey importer's site) but if I were you I would save the extra and spend on the Canon, after all it is a really really good lens. Have a look around the bird forum and see some of the images taken with it by new and older members here who are putting it to very good use.
As a footnote, we bought it for 2 reasons, it came our way reasonably cheaply
and Nikon do not make a lens with the same features as the Sigma full stop.