Which is better Sigma 18-200mm f/3.5-6.3 DC or Tamron AF 18-200 F/3.5-6.3 XR DI-II LD?

If you like most professional and advanced amateur photographers use Adobe Lightroom3, there is an option in the Develop module where you can enable profiled corrections based upon the camera and lens you are using. This takes care of some of the vignetting and colour aberrations with many of the complex lenses used by most photographers. The information used by Lightroom is embedded in each image file.