There have been many discussions around these two lenses. I suggest you read how others responded to you here and also read the in depth and user reviews of these two lenses.
Here is a copy of what I stated on another similar post:
I have 17-70 that I bought new 8 years ago. Optically it is really good especially close range up to 50mm. From 50mm to 70mm the optical quality is not quite as good. The SDM still works, sometimes. I need to use this lens at least once a week otherwise the SDM stops working. Recently I bought a used 16-85 and now can realistically compare them:
1. Build, 16-85 is much better, it is WR and has a more solid feel with none of the barrel wobble in the 17-70
2. Sharpness, 16-85 is better, especially above 50mm but the difference at 50mm and wider is not huge. Both are very good for zooms.
3. Autofocus, getting the 17-70 to work takes patience and time and several manually focused photos until the SDM wakes up. Above 50mm the autofocus is erratic if there is not good contrast and good light. SDM is the biggest weakness of this lens. Regarding the 16-85, it works fine, all the time, Much Better!
4. size and weight, they are about the same.
5. speed, slight advantage for the 17-70 except wide angle where the 16-85 is f3.5.
UPDATE: the 17-70 SDM died, I now use the 17-70 as a paperweight and the 16-85 as my primary walk around zoom lens.
Bottom line, its a no brainer, I would go straight for the 16-85. If cost is a consideration the 18-135 could be a good choice as well.
Read more at:
Question about the 17-70 SDM - PentaxForums.com