Tamron 17-35 or Sigma 18-50?

Hello, This is my first post here after being a long time "lurker". I have searched the forum, but still can't find the answer to my question. I own a 350d, and need something to replace the "kit" 18-55mm lens. I will be using it totally for out door stuff HERE IS A LINK TO THE TYPE OF SHOTS I TAKE) http://www.railpicture​s.net/showphotos.php?u​serid=735
Anyhow, I need to know if anyone has owned or compared the Sigma 18-50 EX DC to the Tamron 17-35DI. I can't quite afford the 17-40L so I think I have it narrowed down to the Sigma or the Tamron. I have researched enough to know that the Tamron 17-35 DI seems to have better optical quality than the Sigma 17-35 EX DG, but I'm not sure if it is better than the Sigma 18-50.

Thanks!
Willie

I guess I would also like to know if anyone has compared either of these lenses to the "kit" 18-55mm lens. I would be using whichever lens I choose for outdoor, sun at my back, & in good weather, so flare is not an issue for me.

I tried out a Tamron 17-35 at a camera store once. The camera was set at AWB inside and the shots produced were great. I would love to pick this lens up.

I have to Tamron 17-35 lens. Apart from slight CA at 35mm wide open it performes great.