Pentax lens upgrade - buying a sigma 17-50mm f2.8 or 10-20m f3.5?

[xenocide72]

Hello.

I have a Pentax k-1 camera with 2 lenses: a Pentax DA L 18-55 F3.5-5.6 lens that came with my old K-x camera and a Pentax DA* 60-250 f4 telephoto zoom.

I am looking to upgrade and i am trying to decide between a Sigma 17-50 mm f2.8 and adding an ultra wide angle Sigma 10-20 mm f3.5

I shoot mostly landscape, nature and outdoor shots. Some action. Virtually no portraits.

I know these two lenses I'm thinking about aren't really comparable so the main thing is whether the Sigma 17-50mm f2.8 is a substantial upgrade on the Pentax 18-55 mm f3.5-5.6. Or should I just add the ultra-wide angle? My thinking is that the if I add the ultra-wide angle I'm only leaving out the 20mm-60mm range that would have to be covered with my current pentax 18-55 mm lens and I don't think I really take a lot of shots in that range anyway. However, I also wonder how much I really need an ultra wide angle lens.