Having had the sigma and now owning the pentax, I personally like the pentax. If I want wider I have the pentax 10-17 fisheye as well (angle of view fits perfectly w/the 12-24, and although skewed I enjoy the view). you should check out this forum post for a lengthy discussion:
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/38480-whats-be...x-12-24-a.html
I find the extra reach of the 12-24 is also significant, and when you go beyond 12mm (down to 11 or 10) w/the sigma the distortion becomes strange to the eye (someone else called it 'disconcerting' in that thread, I tend to agree!)
Having just recently acquired the 12-24 and 50-135 da* i am (probably regretfully!) thinking about putting my 16-45 on the chopping block. It overlaps too much with the 12-24 (especially when I run exposure plot with my current photo collection), and I am thinking I want the 18-55 WR to compliment the 50-135 (don't see a need for the 16-50 - an f2.8 ultra-wide just doesn't make as much sense to me i guess).
back to the 10-20 vs 12-24 debate, it depends on your need i guess. for interiors it is helpful, but to give some perspective you can fit a full size door frame at 12mm from about 4 or 5 feet away; that is wide enough for me 95 percent of the time.