Search for wide angle..

My current lens is the kit lens(Canon 28-135mm IS USM) I got with 40D. Because of the cropped sensor, the widest I can get is ~45mm (28*1.6) :(.
I am looking for a wide angle lens... The shortlisted options are..
Canon 10-22mm
Sigma 10-20mm f/4-5.6
Tokina 12-24mm f/4

I read good reviews about all three lens...
Canon is removed from the list because of its price (~700$).
Now I have to choose from Sigma and Tokina..
Sigma is wide(10mm) but Tokina has fixed focal length..

I am a bit towards Sigma as of now. But still undecided....

My Current Photography Equipments are -->

Primary Equipments
- Canon EOS 40D
- Canon EF 28-135mm f/3.5-5.6 IS USM
- PlatinumPlus 6200DX
- Hoya Polarizer
- UV Filter
Alternate Equipments
- Canon S2IS
- Olympus Stylus 850 SW