Tamron zooms-in/out in the direction of Nikon, and Sigma in the direction of Canon..
it may sound weird but i find myself not favourable in the Canon way of turning the zoom ring.. which was one of the major reasons i gave up using Canon and stick to Nikon..
since you are using Canon cameras, and while other zoom lenses are turning the "right" direction for you, Sigma may be the option..
i haven't got a chance yet to try out the Sigma but according to reviews, image quality is more or less same..
Sigma got HSM though i don't know it may be faster in focussing compared to Tamron..
check this anyway for comparison with Canon/Nikon 18-200 counterparts..
http://www.dpreview.com/lensreviews/tamron_18-270_3p5-6p3_vc_n15/
buying 3rd party products sometimes may face drawback in "resale value"..in case you want to sell them back..
but i have no idea still to sell out my Tamron 18-270 :lovegrin:..it is slower in lowlight than Nikon but 105mm more on the tele end means a lot for me