Have a look at the reviews on
www.slrgear.com and
www.photozone.de - they both test both lenses. I'm not sure what you think your getting moving to the Canon lens ... another 15 mm & IS but its slower in terms of maximum aperture.
I used to have the Canon lens a while ago ... and its not a stellar performer in their range particularly at the wide end where it suffered from severe barrel distortion & CA. Their newer 15-85 is a much better performer - I hired one last year to go on holiday rather than take my 24-105 &10-22 and was very happy with the results... although it is more expensive