I did a quick check of the differences, listed in the following article:
http://www.icewebring.com/rmss-vs-rmfrp/
It seems that there is almost no difference.
Talents are deemed better in the RMFRP version, and you get descriptions for all of them when you purchase ERA RMFRP Character Law.
Some of the everyman/occupational skills for some races/professions may have changed, so you might want to check
http://www.icewebring.com/rmss-vs-rmfrp/17-cultures-races/
and
http://www.icewebring.com/rmss-vs-rmfrp/18-professions/