If you are going to spring for the Sigma, I would save a bit more and get the Canon 10-22. I've owned the 10-22 and used the Sigma 10-20 and the build quality as well as image quality was noticeably worse with the Sigma. It wasn't just a trial run I did with it, either. I had sold the 10-22 and was wide-angle-less for a brief period and borrowed the Sigma from a friend for a couple of shoots.
I mean, I hear Sigma's built quality is spotty, so you may end up with a great model, and I may have ended up with a crap model. Who knows. I don't have an inherent distrust of Sigma, I own their 70-200 f/2.8 and there is nothing to hate about it (except the lens cap doesn't fit snugly for some reason).
Just tossing that out there. As someone said, soft corners is part of wide-angle photography, so I would get as sharp a lens as you can. If you've got $500 now, I would save another $200 or so and look for a refurb or good conditioned used, or maybe you can find one new for around there.
edit: Amazon has the Canon 10-22 for $666 (oooh spooky) which is a steal. That lens is nice.