How to choose between these two wide angle zooms? Depends on your priorities and your application. I decided to update my original post, for my own benefit if not for anyone else's. Don't mean to open up the can of worms again.
Priorities:
1. Wide field of view? Sigma has 10mm versus 12mm.
2. Narrow field of view? Pentax has 4mm more in the "long" end.
3. Money? Sigma is cheaper.
4. Speed? The Pentax at 12mm f/4 is already performing at its best. And it's constant f/4 all the way through the zoom range.
5. Sharpness? The Pentax is sharper towards the edges than the Sigma, unless you stop down to f/8 or so.
Application:
1. Landscapes? If you are careful with your composition you won't find the Sigma barrel distortion at the edges disagreeable.
2. Architecture or other geometrics? Pentax has less edge distortion but the Sigma from 12mm is fine.
3. Interiors? The Pentax is faster and sharper wide open. But if you're using a tripod you can stop down.
Both these lenses have mustache distortion that is not going to be correctable and CA that is.
Jury is still out on other aspects like colour rendition or bokeh.
Future choices might include the Tokina AF 11-16mm f/2.8 if Pentax clones it. This has 2% barrel distortions wide, but highly impressive resolution and might be better for interiors, being one stop faster. There's also the Tamron 10-24mm f/3.5-4.5, currently in development. This will cover the widest range of the four lenses.
Currently the full frame Sigma 12-24mm F4.5-5.6 EX DG ASPHERICAL HSM is also available. It's bigger and 50% heavier. Not too much discussion about it on Pentax forums.
Outside the Pentax box, consider a full-frame camera if your bread and butter is wide-angle.
References:
Pentax DA 12-24mm f/4 AL ED [IF]
test report.
Sigma AF 10-20mm f/4-5.6 EX DC
test report.
Tokina AF 11-16mm f/2.8 AT-X Pro DX
Canon test report.
Tamron SP AF 10-24mm f/3.5-4.5 DI II LD Aspherical (IF)
announcement.