if we're talking old school judys then the differences are as follows:
Judy SL has Ti bolts on fork and crown, an Alu ahead steerer, a yellow paint job. Costs a load more than XC
Judy FSX have carbon lower legs and a CNCed brace that breaks. Costs a little bit more than SL
Other than that there is no difference.
Couldnt tell you about the more recent ones though.