The only valid reason for not getting the 10-20mm, but opting for the sigma 12-24mm would be if you ever intend to use a film body again, or are one of the fanatics that believes that pentax will go to full frame.
I use the 10-20 so often when travelling, because I find if I am touring a city, you can never have a wide enough lens. I used one of the EXIF data analysis tools and checked my 2007 shooting habbits. 20% of all my shots were take between 10 and 20mm (is that justification enough?)
and 12% of all my shots were taken at 10mm. That translates into over 1500 shots in the year taken at 10mm.
This year I expect will be similar.