site is a pretty good one for sussing it all out, but as far as I know the differences are the following:
1. 50mm C lens, non-T* coated, silver cosmetics, sharp at infinity but only decent at close range, not as good of coatings as later, parts are no longer made.
2. 50mm C lens, T* black cosmetics, same optical design as above, better coatings, still will have issues getting service due to shortage of parts.
3. 50mm CF lens, newer style and often preferred mechanics, same optical formula as above, T* coated, parts are much easier to find and for now are still made. Likely the best value if on a budget.
4. 50mm CF, CFi and CFE FLE, new optical design with a user selectable floating element resulting in much better close range correction, the least expensive version being the CF at $600-$800 depending on luck and condition.
Some have said that the focus action on the newer CFi and CFE lenses are easier to pull, but for most of us analog shooters, the 50 CF lenses, either the non-FLE or the FLE represent the best value. That's my take on it at least...