Having just been out for a couple of hours in snow and sleet, I am very, very glad of the Pentax 16-50mm's robust WR. I could not have used a non-WR lens without a lot of anxiety. I've only tried the Sigma in a shop and it seemed a very nice lens. However, sharpness (if there is a difference between the two) is only one criterion. The Pentax lens has super colours, spot on for me. I suspect, albeit without evidence, that any difference between them is less than the difference between either and a quality prime lens.