The Sigma 10-20 has terrible quality control issues. Think it's OK if you get a good one.
Two of my friends have the 10-20.
The first one god a good one on the third attempt (Nikon mount)
The second one decided not to stay with Sigma after getting a second duff one (Sony mount).
FYI - See this thread....
http://www.dynaxdigital.com/index.php?topic=4353.0
and this one (for the response from Sigma)
http://www.dynaxdigital.com/index.php?topic=4405.0
The Sigma 10-20 is not a lens to buy online, unless there is exceptional customer service.
If you are going to buy the Sigm, I would buy from a local dealer so that you can keep popping back and avoid the postage costs.
I have the Tamron 11-18, which was my second lens afer taking the second one back for being too soft. Think the Sigma gets better reviews though.
If it was my money - I would save and want to get the Canon.