Assuming you're talking about the Sigma 17-50 f2.8- The Canon is optically superior, primarily in the image corners, and has superior AF speed and performance.
http://www.the-digital-picture.com/Reviews/Sigma-17-50mm-f-2.8-EX-DC-OS-HSM-Lens-Review.aspx
If you're referring to the 17-70 f2.8-4, then you can also add aperture- the Sigma is f2.8 wide open, but quickly reduces as you zoom in. I can't recally exactly but I think the Sigma is only f2.8 at 17mm and drops sharply with focal length.
Both Sigmas are fine lenses (there are very few modern lenses that aren't) and you can potentially get great results from them. The fact is the Canon is demonstrably better. Whether the differences are important to you is something only you will know, of be able to find out.
I bought the Tamron 17-50 f2.8 and got great use out of it, but when I upgraded to the Canon 17-55 f2.8 the difference was clear. For me, just having IS was worth the price of admission, and changed the way I could take photographs.
