I would recommend to read some independent reviews of the two lenses by
www.photozone.de and
www.slrgear.com. (Both sites are respected by Pentaxians for their independece.)
The Sigma 10-20mm f3.5 HSM was recently tested by slrgear.com: "Sigma has made some dramatic improvements in this constant-aperture version of the 10-20mm lens, and has made them where they count: at the 10mm end of the focal length spectrum, where (arguably) the majority will probably use this lens. The lens is very sharp, though we're seeing slightly more CA than we did in the previous version. [... But ] The original Sigma [10-20mm f4-5.6 EX DC HSM] ultrawide-angle zoom still holds its own, being slightly sharper at the telephoto end of its spectrum than the newer version. CA is also slightly better in the older version, though the newer version improves on corner shading and distortion." (
Sigma Lens: Zooms - Sigma 10-20mm f/3.5 EX DC HSM (Tested) - SLRgear.com!).
Photozone.de tested the Sigma 10-20mm f4-5.6 EX HSM with a Pentax mount, and the lens was praised: "The Sigma AF 10-20mm f/4-5.6 EX DC showed an pretty impressive performance with a few bugs. The resolution characteristic is generally very good to excellent. [...] The CA characteristic (color shadows at the image borders) is generally pretty good except in the extreme corners at 10mm. The build quality of the lens is very high and leaves nothing to be desired. " (
Sigma AF 10-20mm f/4-5.6 EX DC (Pentax K) - Review / Test Report). The same site (photozon.de) tested also the Sigma 10-20mm f3.5 HSM: "The new Sigma 10-20 mm f/3.5 EX DC HSM leaves us with mixed feelings. The build quality of the lens is certainly excellent, distortion is a little high at the wide end (but excellent for the rest of the zoom range), there's also a fair amount of vignetting and CA, but that's typical for this lens class. Resolution in the center is excellent, borders also are nothing to complain about and corner performance is ok at 15 and 20 mm, but very disappointing at 10 mm. In summary, it makes one wonder who's supposed to buy this lens." (
Sigma EX 10-20mm f/3.5 EX DC HSM - Review / Test Report - Sample Images & Verdict)
Both sites more or less suggested that the newer 10-20mm f3.5 was not a major improvement from the older model "unless you really need the speed of this lens, but for those use cases the Tokina 11-16/2.8 is probably the more attractive option" (
Sigma EX 10-20mm f/3.5 EX DC HSM - Review / Test Report - Sample Images & Verdict).
These reviews are worth to read because they suggest some lens alternative options.
Hope that the comment will be useful...