The big differences:


17-85 f/4-5.6 IS:
  • Image stabilization
  • Much longer 50-85mm focal length range
  • Full time manual
  • Name brand



17-50 f/2.8:
  • Much higher image quality (contrast, sharpness, aberrations, distortion, you name)
  • One to two stops faster f/2.8: stop action, thin DOF



For my purposes, the 17-50 f/2.8 is the best choice. I could not live with anything slower than f/2.8, nor with the aberrations of the 17-85 superzoom.