I have a guy where I live, I call him Uncle John because eveyone knows a guy like him - who always gets the best top most expensive gear and doesnt know how to use it. Hes always asking me why my oly is so small and why I dont use L lenses on my canon. To me, part of the reason I got the white Canon telephoto was because I needed it and to shut that guy up and bash him with the lens if he talks to me again in the field. But if you dont have an Uncle John then whos going to care about your lenses if its all Canon, unless the image is rubbish.
Theres nothing wrong with Sigma and Tamron - Personally Id never buy a Tamron becuase I never like the build quality and if I bought one I think it would crumble in my hands but I would deffinately consider Sigma. And a Caveat: Tamron make good lenses - the Glass in one of their 2.8 and 1.8 lenses are fantastic - they make great pics, but I cant buy one because I just never felt confident running etc with them on my camera.
Oh and if weather sealing is important, Id get the canon if thats something your camera body has too that you feel you need.
.. Well, it's true, in the end, it's the photo that counts. I'll do some more research on the Sigma. It seems to be almost a steal for the price. Got to get rid of some other Canon lenses first though, the 28-135 Kit lens and the 55-250 Zoom lens to get the money for the Sigma