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When I wanted an ultrawide lens for my 7D I tested a bunch out, including the above mentioned Canon 10-22, Sigma 10-20 and Tokina 11-16. Ultimately it came down to the Canon and Tokina, and for me, it was no competition... the Tokina 11-16mm 2.8 was amazing.
I felt the IQ as quite a bit better, plus it's 2.8 throughout. Of course you're giving up some range, but for my purposes, I didn't like the fact that the Canon seemed to jump to 4.5 almost immediately after 10mm. However, if you're shooting real estate with a tripod, you're probably not needing a fast lens.
Finally, you mention weight as a factor... and the Tokina isn't a light lens... but for me that was a plus. It felt like a much more quality lens than even the Canon.