I've researched these two lenses recently for use in my studio now that my trusty old Tamron 28-75mm died. Optically, I found what has already been stated on here: both lenses provide excellent image quality, so the decision comes down to the slightly different feature set of each lens, and of course the price.
I liked the idea of the longer focal length on the 16-85mm since a narrower field of view lets me get more of my subject and less of the background when I'm doing product photography. However, the Sigma is the class leader in close up/macro work, which is probably why they use "Macro" in the name. It has a minimum focus distance of 8.7" (22 cm) and a maximum maginfication of 0.37x. The Pentax, on the other hand, focuses to 13.8" (35 cm) with a magnification of 0.26x. So depending on how one plans to use the lens, this could be a factor. For me, the close-focusing is a big advantage of the Sigma in my studio, but for somebody wanting to do a little quasi-macro work of flowers and such with their walk-around lens, it would also be useful.
The Pentax goes wider, so if you don't already have an ultra-wide lens, that may be compelling. And of course it has WR. For my uses, those were both non-issues. The Sigma is slightly faster, but for me in the studio that was also a non-issue since I'll usually be stopped down anyway.
In the end, the lenses are more similar than they are different, with a few key differences, so it's kind of a tough call. But the $150 price advantage for the Sigma could tip the scale for a lot of people, assuming they don't need WR.
Last edited by Edgar_in_Indy; 10-30-2015 at 08:30 AM.