My opinion is biased as I love walk-around and 1-lens to do-it-most with my
Sigma 17-70mm f/2.8-4.5. The Pentax DA may have better overall image quality. The
SDM is nice to have while I consider the weather sealing a bit more important to have in a starter and do-it-most kind of lens. The focal range in 17-70mm is wonderful and that range will fill your needs better as a
single lens for broad usage as compared to other similar lens in
18-55, 16-50, 16-45, 17-50, 24-60 and 28-75, I like the range in
17-70mm the best for versatility. I think the Pentax DA will be more expensive for you to factor into the equation on cost vs value. In the price of the DA 17-70mm, you are more likely to get a used copy of Sigma for ~$300 and use the difference to finance a used copy 50mm such as F 50mm f/1.7, FA 50mm f/1.4, or a good manual focus 50mm such as Pentax A 50mm f/1.7.
I wrote about the
Pentax 1-lens choices in which I pick the Sigma 17-70mm f/2.8-4.5 as my center of focus as the most versatile candidate for walk-around purpose. The build is excellent in the Sigma 17-70mm.
AF is reasonable but loud as usual when compared to
SDM.
I also have
Tamron 28-75mm f/2.8. Image quality is better in the Tamron but in terms of versatility and flexibility given the 1:2.3 close, the winner of choice goes to the Sigma for walk-around and kit's lens that can do landscape well in 17mm, a great travel and street shooting lens throughout the zoom, a great portrait zoom in the 40 to 70mm range, a great close up for flowers in 70mm with 1:2.3 close up ratio. The lens works out great for me in traveling.
Regards,
Hin