You may wish to read some relevant threads at both DPreview and PentaxForums.com: [
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/84325-zoom-lens-k-7-a.html].
There is a solid review of the Pentax 18-250mm at [
www.photozon.de] : [
]Pentax SMC DA 18-250mm f/3.5-6.3 ED AL [IF] - Review / Lab Test Report.
A review of the Sigma 18-200m is at [
]Sigma Lens: Zooms - Sigma 18-200mm f/3.5-6.3 DC (Tested) - SLRgear.com!.
A review of the more recent and more expansived Sigma 18-250mm OS HSM is at [
]Sigma Lens: Zooms - Sigma 18-250mm f/3.5-6.3 DC OS HSM (Tested) - SLRgear.com!.
I have the Pentax 18-250mm and I am very happy with it. I understand that the Sigma 18-200mm is an older, less performant lens. The newer Sigma 18-250mm OS HSM seems to give excellent results, but for a price.
Hope that the info will assist.