When I was tackling with the same question I found a few side-by-side comparisons by googling. I ended with the Sigma and love it. I came to the conclusion - as most reviewers had - that the drastically higher price for the Canon wasn't worth the moderate increase in quality. Lens distortion at the widest setting with the Sigma is a bit too much, but otherwise it's a fine lens, especially for its price. Build quality is excellent, it handles well and it's compact.