I've just got the Sigma 10-20mm lens, and it's very wide. Seems optically very good, and cam be had for £100/200 quid less than the Canon (although the Canon is slightly better). It depends how much money you want to spend on a wide lens. Personally I went for sigmas offering as it gave me more money to spend on ND grads and the like, which are important when doing landscapes as the lens is.
You might find that while the 17-40 is a cracker of a lens, on a cropped body you might find yourself wanting wider.