As a real estate photographer, I take hundreds of pics every day and so live and breath ultra-wide angle.
I have the Pentax 12-24, Sigma 10-20 f4-5.6, and Tamron 10-24. Generally speaking all three are good lenses but I think the Pentax is the best. It has the least barrel distortion of the three and clearest and sharpest pics edge to edge.
However, sometimes the Pentax 12-24 is not quite wide enough. That extra 2mm that comes with the Sigma and Tamron lenses is very significant in SOME situations. Most of the time the 12-24 is sufficient though.
My least favorite of the three is the Tamron. The further away you get from the center of the image, the more fuzziness you see. I've compared images side by side between the Tamron and Pentax (pics of the exact same scene) and the Pentax wins hands down.
Both the Tamron and Sigma lenses has a little more barrel distortion than than the Pentax but that is easily corrected in Photoshop. That extra 2mm width might be contributing to that extra barrel distortion.
A sample photo for each lens is below. No obvious winner from these photos which just means that any of these lenses will do a good job for you.
Tamron 12-24
Pentax 12-24
Sigma 10-20 f4-5.6