With the same zoom (18-200) and speed (f3.5/6.3), which is the better lens, Tamaron or Sigma?

Looking for a low cost lens form my Canon digital camera.

They are both comperable, the tamron will be cheaper [only a bit], but both too slow for my liking. IMO, you're better off spending $50-$100 more and getting a canon lens of equivalent focal lenth that is, at the very least, faster at the long end than f/6.3 [they're not all that much cheaper, either way you're spending $300-$400 anyway]. Personally, I would never buy a slower third-party lens for the sake of saving those 50-100 bucks over a lens designed with the camera, not just for it, in the same general price range. Don't get me wrong, sigma and tamron both make good fast aperture lenses, but where quality counts in a slower lens, I'd rather stick with the manufacterer of my camera. If you can live with f/6.3 on the long end, sure go with either one. Sigma's DC series lenses seem to be a bit more sturdy, but a few dollars more usually.

~EDITED~

I just had to add another note, for the sake of arguement when I read the answer below mine. Optics? No photographer I have ever known gets the specs on optics, to that extent when choosing good glass, least of all anyone other than a pro. As a matter of fact, the manufactuerers of lenses don't even make that information available, and if it were, it is way too much to research based on simply the need to know. And as far as that is concerned, you don't need to know that information to buy a good lens. I'll agree that reading the reviews, as the last answerer suggested is always best, but "fluor" is utterly moot and of no use to you, unless you are an optics engineer. If you want to go into all of that, be my guest, but if you want to be an informed consumer, do like the rest of us [real] photographers and read reviews, keep your reciept, take some shots, and determine on your own whether the lens you buy is good for your needs.

I have a Nikon digital with an 18-200 Tamaron lens that can do portrait and telescopic work. It is sharp and well made. I don't know if Cannon will use this lens but a camera store would be able to tell you.
Spartawo...

I think if you really want to get it right, forget about the brand names. Start getting into the Optics, or if you like the glass that you'll be shooting through. There is a mass of glass info out there, UD Glass,Mrc Coatings, Fluor Crown, FCD10, Barium Crown, LCB3N. All of these are forms in lens manufacture. The less pure, the cheaper the lens, to a top shelf say for the purposes quality, some Carl Zeis or Schnieder lenses are more expensive because of the clarity and precise nature of the optic glass used. Higher up the scale, faster the light refraction with less distortion.

Have you loooked at reviews for those lenses?

Do a search on "Tamron 18-200mm review" or Sigma 18-200mm review" You'll get a lot of places that do lens reviews, some with sample pics and everything.

Below is what I found just from doing the "tamron" search.