The Sigma 18-125mm HSM OS has lens optical stabilisation (OS). The lens OS is redundant for modern Pentax dSLRs with in-camera image stabilisation (IS)/shake reduction (SR)
The lens OS does not work with the in-camera IS. That is, either you use the lens OS and switch off the camera SR, or switch off the lens OS and use only the camera SR.
The lens was tested by Photozone.de at:
Sigma AF 18-125mm f/3.8-5.6 DC HSM OS (Canon) - Review / Test Report
Its focal length range is shorter than the DA18-250mm (Tamron 18-250mm).
The price of the Sigma is quite more expensive than the price of a 2nd hand DA18-250mm. There were a couple of DA18-250mm/Tamron 18-250mm for sale as used lenses at adorama.com and keh.com over the last couple of weeks. The prices were around US$360.
Hope that the comment will help.