Philip Tam , Apr 23, 2010; 05:01 a.m.
Single focal length lenses tend to producer sharper, less distorted images. They may have larger aperture abilities. In the two lenses you talked about for example, the zoom lens has a f/5.6 aperture when you zoom out to 300mm. The fixed 300mm lens can obviously do f/4, one stop better, at this focal length.
The "L" in f/4L is Canon's way of designating it as a high quality lens. This does not mean however, that the 70-300mm is a bad lens. In fact, generally speaking, for most people, it's the better choice since it's more versatile. A telephoto prime (fixed lens) at that focal length is really only useful in a narrow set of circumstances, in my opinion.