Sigma 18-200mm F3.5-6.3 DC OS
OR
Canon EF-S 17-85mm f4-5.6 IS USM
Which is better in terms of producing a sharp quality photo? I heard the Canon lense is 色淡, and 成像一般... is that true? Does this Sigma lense do better than Canon? Also, which one is better in taking night photos?