Why not keep both with one as a backup just in case something happens? With the way prices for FD lenses(and most manual focus lenses) going up because of m4/3 cameras, you might end having to pay more in the future.
Just picked up the FD 300mm f/4L about a month ago and have been testing it against my non-L. I gotta say, the L is MUCH better in contrast, saturation and sharpness until at least f/8.