From my personal experiences, I've always had a rule of thumb: Buy Tokina for wide-angle lens, and go for Sigma for telephoto lens. I owned a Sigma 28-70 f/2.8 EX, but sold it a month later because it gave me horrible artifacts and exhibited huge amounts of image distortion. I now own the Tokina 28-70 f/2.8 ATX-PRO II, and a Sigma 70-200 f/2.8. You should definately check out Tokina.