Canon 17-40 f/4 vs Sigma 10-20 f/4-5.6 EX DC

I decided to make a choice between Canon 17-40 f/4 and Sigma 10-20 f/4-5.6 EX DC HSM. It’s pretty tough, both lenses look good and I know that many people would recommend one or the other. So I just wanted to get your opinions and experiences if you have any of the above. If you can include some pictures and describe pros and cons I’d appreciate it.

I have the Sigma 10-20, its a great lens for superwide angel. Distortion at either end is almost nil. The Canon 17-40 really isn't a comparable lens, its wide angel to standard zoom. The Canon 10-20 is a fantastic lens though, allot of money though.