Sigma 10-20mm - which version?

I see that this lens comes in two versions - the f/3.5 and the f/4-5.6. Is the difference in performance for the fixed aperture of 3.5 worth the price difference of $100 or more?  I know the lower aperture is better for low light, and some of our shots will be indoors, but the lens will more often be used in daylight for landscapes and architecture.  I've actually read that the f/4-5.6 is sharper despite the lower price, but that was from just one source.  Has anyone used them both?  Thanks.