The Sigma has an OS version of the 18-200mm which is better than the non-OS version in terms of optical quality. And the OS for sure help in a situation where you need a slower shutter speed.
As for the optical quality, according to the web reviews, the 18-200 OS lens from Sigma is not tack sharp. The optical quality depends on the what focal length and f-stop you are shooting with.
If I am in the market of buying the 18-200mm lens for Canon, I will look at the Sigma OS version. And of course, wait a little longer and see how well the new Canon EF-S 18-200mm F 3.5-5.6 IS lens is. So far, the price/optical quality of the 55-250mm EF-S lens is pretty good. Hoping that the new lens is as good as the 55-250mm.
The recent issue of American Photo magazine named 55-250mm EF-S lens the best buy.