Upgrading lenses?

I have the canon rebel xt and I am looking to upgrade my lense. I have narrowed it down to either the Sigma 18-200mm with optical stabilization at about $325.00 or Tamron 18-200mm at about $259.00. Which company makes a better lense and why? Also which one of these lenses is a better value?

or is it worth it to step up to the Tamron 18-270mm VC even though it costs about twice the price of the other lenses?