For the best comparison of these two lenses, go to the the-digital-picture.com You can in the 12233 chart put the two lenses side by side and look at sharpness and such... the 24-70L outpreforms it, especially in the corners, on all focal lengths... center sharpness tends to be a wash/maybe even a slight edge at times with the sigma, but the corners fall apart. Lastly, on their summary of the lenses, they said if sigma was the same price as canon, NOBODY would buy the sigma, however since it's half the price, it's something to consider. It also mentioned if you get flare, even a little flare, you lose a ton of contrast in your entire image.