The Sigma rates better on Photodo. You might query Photozone also.
According to Photodo, the Sigma is actually sharpest at the long end, while the Tokina is sharper at 80 and 135 than at 200 -- but not by much.
You will probably be pleased whichever you buy. The Sigma, though, has developed a reputation as one of the legendary lenses. Incidentally, Sigma has announced that they will bring out a "digital" version of this. When?? Will it be available in Pentax mount?? We don't know, so I don't know whether it is worth your while to wait.
Joe
