Parts are no longer available for the C lenses. It's worth the extra money for the CF in case something where to happen to the lens.
David, are you sure about this? I had my C lens SWC/M overhauled in 03 in the New Jersey location. At that time I was told that the only parts that were unavailable were cosmetic pieces for the silver C lenses, and that shutter parts were abundant, and the whole situation was overblown.