Kevin,
From what I have read, the various 50/1.4s have very similar optics (if not identical), with somewhat different optical coatings. The build is very different though - the older M and A series are all metal, while the newer FA is plastic. The real difference though is that the FA gives you auto-focus; A is manual focus, but it does have the "A" setting in the aperture ring that lets you use the lens in aperture priority and other exposure modes; the "M" and "K" is manual focus and requires stop-down metering.
And then there is the extremely rare "F" 50/1.4 lens that some people believe is far better than any other pentax 50/1.4, and should fetch $500+ on the used market!