Sigma 10-20 Vs Canon 10-22

Ciao belli,

after long time of thinking, discussing, waiting and saving money I finally
bought a 550d with 18-135 and im really happy!

Now I want to buy a wide angle lens but I dont know which one.

I first thought about the sigma 15mm fisheye but because of the high price
and the fact that is too special to be a second lens I decide
to go for something more usable.

So now Im thinking to get either a
sigma 10 20 (3.5 or 4-5.6???)
or a canon 10 22

Any suggestion?
Alternative?

Thanks