Chris -
Excuse my n00bness to the wonderful world of digital slr's, but what is the difference between USM and EF lenses?
I might be wrong, but all lenses made by canon are either "ef" or "ef-s".. The "usm" stands for "ultrasonic motor" which is the technology for their auto focus system. Some lenses have it, some do not. It is supposedly faster and quieter than their other auto focus motor systems.
for instance.. here is the description of the EF 75-300mm f/4-5.6 III from Canon's webpage...
""The optical system, construction, and exterior are the same as the EF 75-300 mm f/4-5.6 III USM’s. The difference is that it uses a DC motor instead of a USM to drive the AF.""
as opposed to the EF 75-300mm f/4-5.6 III USM, which has this........
"" Compact and lightweight 4x telephoto zoom lens ideal for shooting sports, portraits, and wildlife. The newly developed Micro USM makes autofocusing quicker and quieter. The improved zoom mechanism also makes zooming smoother. The front part of the zoom ring now sports a silver ring for a luxury touch."""
I hope this helps!!