Originally posted by Rondec What he said. Unless you are shooting professionally and need a lot of back up equipment, you will find yourself frustrated with the image quality of the lesser lenses over time and end up shooting less with them anyway (at least that is the way I found it). My 16-50 is my most used lens, followed by the 50-135. I love primes, but the flexibility that zooms offer is great and these two offer great image quality as well (and truthfully, they aren't that big either).
Thanks for your input.
I have decided to keep the 16-50. Sell the Sigma 17-70 and keep the 18-250 as a vacation lens. I have rearranged my lenses to be almost all Pentax now and I am really liking the colors that the Pentax lenses produce vs the Sigmas. I know the 16-50 is larger than the others but I still have the 18-55 if I need small.
I was in Florida last week with the Sigma 17-70mm and my KX. It was my first time traveling with the KX and I was very suprised at the low light/high ISO capabilities of the KX. With the extra stop at the long end, the 16-50 should be even better..... I might make the 16-50 and the 50-135 my domestic travel kit in the future....