In comparing the two series of lenses, I do somewhat prefer the old FD type, for a couple of reasons. First, and foremost, the FD series tends to be a bit faster than the newer series, and are somewhat more compact. The origonal style bayonet mount of the FD lenses, is more secure, and is self tightening, as opposed to the lock mount on the EF, and newer FD series lenses. There is also the all metal construction of the FD mount lenses. Though there are some metal mount EF lenses, most are constructed with plastic barrels, and mounts. AS if all of this were not enough, I do not care for teh auto focus feature. Except under a few conditions, it is easier and faster to focus manually. I also do not enjoy trying to fool the auto focus feature, under condition in which my main subject is not always where the camera "thinks" it ought to be. I have similar misgivings about auto exposure systems, and generally have both manual exposure, and manual focus set on my cameras, in most circumstances.
    The older lenses are still available in quantity, and are lower in cost than the EF series lenses. I would love to be able to use them on my new EOS series cameras, but there seems to be no way to convert them, except for some T mount lenses, and through the use of stopped down metering. This is a real shame, and I would probably not own any EOS gear, if it were not for the wonderfull Canon D30 camera body (and the even more wonderfull, or so I have heard, D1). If there were a way to convert my old, metal, match needle F series cameras, or plastic bodied A series to digital, I would use them in preference, even to the remarkable D30.