In that price range Canon has 3 offerings for you:
100-400mm IS L - offers you a wide zoom range and image stabilization. This lens is good quality and the only canon one to give you a zoom range - however optically speaking its the weakest of the three at the long end (it is of course still L grade and thus, whilst the lowest of the 3 its still high quality). This lens has had a troubled past and quality control on it (and mostly any other superzoom lens) can be a bit iffy - its much better now than in its early days of release however and most copies are good - if you get a softer copy sending your camera into canon with the lens should resolve the issue (and under warranty).
300mm f4 IS L (+1.4TC) - offers IS and a good focal range. This is the prime choice if you want the IS support, either bare as a 300mm or as a 420mm with the 1.4TC its the next up on the quality list (at the 420mm mark).
400mm f5.6 L - The top quality in terms of sharpness at this price range, though it lacks the IS (a monopod/tripod makes up for this easily) it has a longer native reach and really is the optically best at the 400mm mark.
In short you've got to choose the right tool to fit your needs - if you want a zoom the 100-400mm; if you want a sharp prime with IS the 300mm+TC and if you want the longest and sharpest and don't worry about zoom nor IS then the 400mm f5.6.
Far as I know the sigma offering is about on par with the canon 100-400mm in terms of optical quality (100-400mm might have the edge, but after processing won't be that different if at all). Note also that whilst its listed as a 500mm the max range at most distances is closer to around 450mm, which is not a big difference from 400mm bare.