Wide lens advice!

Hi I decided to hold on to my 50D in lieu of a 60D with some great advice from our forum members - Thanks again! Now I'm thinking of buying a wide angle lens! Currently own the Canon 24-105L and Canon 50mm1.8MarkI.

I got my 50D last year and I'm slowly expanding my lenses and equipment as I evolve/learn.

Do I save up (a couple of months) for Canon 10-22mm $500-$680 used to new, without hood or buy a Sigma 10-20mm for around $300/350 on ebay?

I also heard good things about the Tokina 11-16 - but price-wise its in the Canon 10-22 range!

Im leaning towards, saving my money and getting the Canon 10-22mm wide but any suggestions or advice would be very welcome and appreciated!

I'm a casual, student of photography my subjects include my kids and family, architecture, outdoors, portraits, and I want to take more pictures at work - airplanes, mountains, etc (I fly for a living) - that is why I want a Wide!

I recently purchased a Canon 580ex II flash - so that is why I'd have to wait for my next purchase.