I put the Pentax k-x to work on a nice scene in the yard. First up was the DA 50-200 WR at 70mm for a wet-leaf image, then the Sigma 17-70 (gen.1) - both at f/6.3 but auto WB/iso. Looking at them overall I would take the 50-200 by a hair - but of course it's in its midrange vs. the extreme end so to some degree that's an expected outcome. The background on the lower image is slightly more blue perhaps, and while exposure info is a match the DA-wr chose iso200, the Sigma 250. I prefer the Pentax' choice. It should also be noted that AF was engaged, and on the k-x one cannot be certain it chose the same focus point when multiple points are in play (I had AF-5point set). When compacted by Picasa's collage tool it is very hard to pick a winner.
OK enough suspense: the top image is the 50-200.