I went for the Sigma 10-20mm f4-5.6 EX DC HSM, I tried out the Sigma 10-20mm f3.5 EX DC HSM version before buying but image quality was indistingushable from the cheaper model and I couldnt justify the extra cost. The f3.5 also has an 82mm filter as opposed to the 77mm and that bigger filter size is just a pain and moves you into COKIN Z filter territory (expensive) and substantially dearer round glass filters as well.
The constant f3.5 aperture is to my mind not a selling point on such a wide angle lens as the DoF is huge anyway at all apertures compared to telephoto where the narrow DoF means every f stop and half stop counts for keeping shutter speeds up to eliminate camera shake and maximising what is generally a narrow DoF anyway. These things just aren't an issue at the wide end.