The Sigma 10-20mm (fantastic lens) does not cost as much as a Canon 10-22mm. Other than that, 18mm will work in "most" situations. Just depends on if you want to buy another lens or not.
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Canon 50D: Sigma 10-20mm f/4-5.6 EX DC HSM , Canon EF-S 18-55mm f/3.5-5.6 IS, Photoshop CS5
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