Generally, lenses with this wide range are not recommended to anything beyond amateur and novice photography but the temptation to use one lens for a wide selection of situations is high. If you are concerned with the quality of you photos you are better off with a set of two or more lenses to cover this range (for example, 18-50 and 70-200). See the excellent faq at photonotes.org. If you don’t mind compromising quality in favor of flexibility, get the 18-200.

Canon doesn’t have 18-200 lens, the closest is Canon 28-200 but if you willing to compromise the wide end, also take a look at the Tokina 24-200 which gets good reviews.

As for 18-200 Sigma vs Tamron, I don’t think there is a clear winner. The opinions vary in both directions. Just read the reviews and decide for yourself:

Tamron AF 18-200mm and Sigma AF 18-200mm

Hope this helps.

- Paul