i've used the canon EF fisheye quite extensively and i hear the FD is better. considering the canon EF fisheye is amazing in quality, it differed very slightly from the sigma i also used.
i hear the FD and EF lenses put out by canon have extremely minimal differences except that the FD lenses were stopped being made because of "environmental reasons" (ie. using lead in their production). they're much more solidly built and i've even heard that they are of better quality from some self-proclaimed camera techies.
my roomate 2 years ago sold his sigma to buy the canon and didn't regret it in the least. he felt the extra money was worth it because it was his most widely (excuse the pun) used lens as he was a skateboard photographer. if you're only using it once in a while, i would save your money and go with a sigma. the lighting, film, shutter speed, aperature setting, using/not using a tripod and the speed of your subject will determine the quality moreso than the actual lens.
i think what it would boil down to is: 1. how much are you going to use it? (is the $200+ extra going to be worth the amount of photos you'd produce from it?) 2. what f-stop are you going to use? (at f8-16, i will bet you'd never be able to tell the difference between them).
if you want to save even more money still there's the zenitar fisheye that i've used before with good success. even ken rockwell (i know he's a digital nut, but i trust his opinion on the technical aspects of the lenses) says "If you stop this lens down to f/16 and are prepared for 1950's style manual operation on a full-frame or film camera you can get better results than I get with my Nikon 10.5mm fisheye". the nikon lens is a new addition to the nikon digital cameras, so this is a comforting thought. it also comes with a uv, red, yellow and yellow-green filter set too, which is pretty rad.
