To answer your question, I have the DA 16-45. It is generally regarded as an excellent lens, and that has been my experience. I recommend it without hesitation.
I have not tried the Sigma. It is said to be based on the 35 mm. Sigma 28-70 f2.8, which was not the best in its class, especially wide open. That said, the June issue of Popular Photography reviews the Sigma 18-50 f2.8 very favorably. The SQF numbers suggest quite good performance wide open, as well as at other apertures. So perhaps Sigma tweaked the optical formula. On the other hand, sometimes Popular Photography awards good reviews to lenses that turn out to be mediocre, if you have a critical eye.
In the cases where I have done critical comparisons of top-flight Sigma lenses against top-flight Pentax lenses, the Pentax wins. Sometimes, though, you have to go to a great enlargement to see this.
For low-light photos, you might want to consider some prime lenses. These will allow you to shoot at lower ISO, which will improve image quality. Depending on your finances, a nice combination might be the DA 16-45, FA 31 f1.8, FA 50 f1.4 (if you can find one), and FA 77 f1.8. For wider views you could add the FA 20 f2.8.
When you are photographing a concert, are you singling out an individual, or the entire orchestra? If the latter, you need wide angle. If the former and you can only afford one lens, make it the FA 31.
Joe
