The differences? The Sigma is less resistant to flare. The Sigma shows more CA. The Tamron is noticeably wider (17mm vs. 18mm - and even wider than the Canon with the same 17mm designation). The Tamron focuses faster, the Sigma focuses more quietly. The Tamron focuses much more accurately - My keeper rate is far higher with the Tamron. My Sigma has a weak left side - For sharpness, the Tamron beats the Sigma easily on the left side, but the Sigma is slightly better on the right. The Sigma rings rotate the Canon direction, the Tamron rings rotate the opposite direction. The Sigma is less expensive (but not by much). The Sigma's requires its lens hood to be removed to install/remove the lens cap - this is a pain - Plan to buy the Tamron cap if you purchase the Sigma lens.