Originally posted by UncleVanya The way to select between them is summarized this way in my opinion:
Those are excellent points. I should add some more points :
1) Do you already have the 55-300 PLM or another high quality telephoto (like the 50-135 or 60-250)? Choose 16-85.
2) Do you have the 11-18 or another high quality wide angle (Sigma 8-16, Sigma 10-20, Pentax 12-24), but no excellent telephoto? Choose 18-135.
3) Do you have 62mm filters, or do you need high quality filters without spending too much? Choose 18-135.
4) Do you need the best sharpness, especially off-center? Choose 16-85.
5) Do you dislike changing lens unless absolutely necessary, and like to zoom in? Choose 18-135. Even if I love my 16-85, and use both 16mm (and more often) and 85mm, I feel the latter a bit too restricted.
Another possibility to consider is the Sigma 17-70. It is not as wide as the 16-85, nor telephoto enough as both Pentax lenses, nor WR, but its distinctive feature is to be faster F2.8 at 17, and F4 or F4.5 at 70, depending on version.