Both lenses can be good and both can be bad. I have seen excellent images from both lenses. I was looking at both of these last year, and ended up with the Sigma. If you need to send either in for calibration, they usually come back great.
The Sigma that I have is really sharp, fast and accurate. This is my main lens on my camera now. I do wish it was a bit wider, but if I need really wide I get my 10-20mm out. Mine is really sharp at 2.8, and just a bit better when stopped down. As for the noisy auto focus on the 24-70, it does not bother me. I have heard worse, but it is no HSM or USM lens for quietness. I wish it did have HSM on it, as the pull and turn manual focus is a pain in the butt.
If you dont mind me asking, what type of work will you be using this for? Do you think you will need more on the wide end or the long end? Also, what camera will this lens be used on?
Also, what other lenses do you have? Do you have CP filters for these, as the 82mm on the Sigma can be some big bucks for a multi coated one. Price wise, these lenses are about the same, and both give great quility images.