The Canon 17-55 is an expensive lens. If you can live without the really wide-angle and fast aperture, I can strongly recommend the Canon EF 24-105mm L IS USM. It is about 50$ more expensive than the 17-55mm. I almost never find it to narrow and it would definitely exploit the potential of your lens. (Note that this lens is also quite big and heavy). Just a recommendation from my side
I do not have any experience with the Sigma, so I will let other more qualified members answer you concerning that.