If you had a choise between three lenses in the 2.8/70-200mm range, the Tokina AT-X Pro, the Sigma EX APO and the Pentax FA,  in used, but good or excellent condition, which one would you choose. If the price range was about the same. Well, I was not offered a used Sigma lens for Pentax. I tried a new one. I ended up chosing the Pentax over the Tokina. Mostly due to better sharpness and / or better focusing ability, partly due to a smaller focusing gear. Further more the Pentax allows me to switch from AF to MF with only one touch on the lens. Changing mode on the camera body is not necassery. Below I have shown photographs similar to a concert shot situation - a photograph done eith a Pentax prime (SMC K 2.5/135mm combined with a 1.7 x AF adapter -  compared to a photograph done wirh the Tokina AT-X Pro 2.8 80-200mm. The shutter speed is crutial to me because of the subject movements, even though the same speed will give me less DOF with the Pentax combo, since the actual F-stop will determine (optical) sharpness and DOF.