The 500ff Stands for "free form" and has the reflector doing the beam dispersion, while the regular 500 has it on the glass. Think of a 96-98 "frosted" headlight compared to the 99+ headlights. Same basic design concept.
I know for a fact that the 500 has a wider beam pattern than the 500ff. There's a display at a local offroad shop and it's definitely a different beam pattern. I've heard people say that the older regular 500 is better for H.I.D. conversions because of this. Hope that helps answer some questions.