I really like the 17-70 range of lens on APS sensors, it saves a lot on lens changes, esp for the live theatre that i shoot.
I went with Pentax 17-70 F4 for these reasons:
A. SLR Gear.com:
Quote: The lens uses a constant ƒ/4 aperture compared to Sigma's variable aperture;
it's much sharper used wide open than the Sigma. Chromatic aberration is
somewhat problematic at the wide and tele ends of the focal range, much more so
than the Sigma. Corner shading is less pronounced on the Pentax,
B. Photozone.de:
Quote: The Pentax DA SMC 17-70mm f/4 AL [IF] SDM is undoubtedly the most desirable
standard zoom lens in the current Pentax lens lineup. The resolution
characteristic is very good without any significant weakness....The fast and
near-silent ultrasonic AF is also a very welcome new feature.
C. Adams recent poll on SDM shows marked reductions in reported SDM problems. I'm convinced that Pentax knows what the problem was and has corrected it. In any case, i've had the DA300 for 3 years, the DA50-135 for 3 years and now the DA 17-70 for one month :-) - no sdm problems with any of them. To me, this is a past issue, but i totally sympathize with those that were caught by it. (Pentax was less than helpful about the issue)
D. Macro capabilities: the Sigma 17-70 latest focuses down to 22cm while the DA 17-70 goes down to 28 cm, prize goes to Sigma for this feature, but 28cm still isn't bad.
I think both lenses are good candidates for this range, Pentax 17-70 went up about $100 over Sigma in the recent price hike which would probably cause a lot of sales to shift to Sigma. I look forward to the promised Pentax 16-85 - should be interesting.