Whats the difference between these two lenses? Canon EF 75-300mm f/4-5.6 III USM Telephoto Zoom Lens for Canon SLR Cameras and the Canon EF 75-300mm f/4-5.6 III Telephoto Zoom Lens for Canon SLR Cameras. All that I see the that the USM is missing from the second one. What is USM? And is it better to have a lens that has USM?