IMHO get the Tokina - do not pay the extra money for the Canon 10-22
unless you need the extra 2mm and/or can't live without USM. The 12-24mm
Tokina build quality is superior as is the optical qualtiy. I have both
and prefer the Tokina.

Canon 10-22 test:

http://www.photozone.de/8Reviews/lenses/canon_1022_3545/index.htm

Tokina 124 test:

http://www.photozone.de/8Reviews/lenses/tokina_1224_4/index.htm

I like mine (used on a Fuji S2); but the Canon 10-22 wasn't an option
for me (Nikon mount).  Sigma makes a 10-20mm that's getting good
reviews mostly, too.  

I have a 17mm lens from my film days, that I found quite useful now
and then; the 12-24mm doesn't quite get me back there for digital.
But close enough, at least for now.  Maybe an 8mm some day, when
somebody makes one :-).

I'm very pleased with the results I get from my Tokina, anyway.