Hi Fengwei, what is your opinion if you were to compare the 18-50 to the sigma 17-70?
It's 16-50, not 18-50 (16mm vs 18mm is a big difference)
It's hard to compare these two lenses. The DA*16-50 has a constant aperture of F2.8, and it is weather sealed, with SDM, and it's 1mm wider (a bid deal to some people, but not to me). While the Sigma DC 17-70 has a variable aperture from F2.8 to F4.5. At 50mm, it's about F4, that's at least one full stop slower than the DA*16-50 at the same focal length. Not a big deal if you are shooting under good light condition, but it's very important for some people. Of course the Sigma 17-70 is 20mm longer at the tele end, very useful if you tend to use this lens for some portrait work.
I'd say these two lenses are targeted at different users. If you don't need weather sealing, and don't need the extra 1mm at wide end, go for the Sigma 17-70. But if you do take photos in bad weather or around beaches, there is no other choice but the DA*16-50. Being weather sealed, I'm guessing this DA*16-50 lens will be quite resistant to dust too, which is another weak point for the Sigma 17-70 which attracts dust quite easily by many users reports.
Of course, there is a price difference you need to consider too. The DA*16-50 is an expensive lens for most people, but well worth it imo
I have the Sigma 17-70 (a very good sharp lens), and I'm replacing it with the DA*16-50 due to its weather sealing, SDM and being a Pentax
Cheers!