Congrats on purchasing a lens that is very popular around here. I myself have never been able to get too excited about it for some reason, but I've noticed that it is well-loved around here (to say the least!).
When I was purchasing a wide-angle, I chose the Sigma 10-20mm f4-5.6 after extensive research. The combination of very good image quality and the relatively low price made it very hard to pass up. If I would have thought the f3.5 version was a better lens, then I would have bought it, but from everything I saw and read the f3.5 version made too many compromises in image quality compared to the f4-5.6.
I have been absolutely delighted with my copy. It is very sharp, and the ultra-wide 10mm opens up all kinds of photographic possibilities for me that were not available before. Here's a picture from my 10-20mm shot at 10mm on my Pentax K-x at an auditorium where my daughter was having a dance recital. Click on the picture for the full-size 12MP image:
If for some reason the 10-20mm was not an option, then I would have a very hard time choosing between the Sigma 8-16mm and the Sigma 12-24mm. The 8-16mm is the widest zoom available for APS-C and has has incredible image quality for such an extreme focal length range. The Sigma 12-24mm, on the other hand, has a focal length range that would make it an option for a walk-around lens, and it has the added bonus of being compatible with full-frame (if Pentax ever releases a FF camera).
But as Vagrant said, the safest course would be to buy
all of the lenses mentioned in this thread.
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