In my opinion, RM2/RMC is simpler in some ways (e.g. skills) and there is lots of optional material to choose from.
RMSS/RMFRP is better organized and better balanced, the various pieces work together with less fiddling. Character generation is more complex, e.g. with skills and skill categories, and training packages. (I think actual play is basically the same complexity.)
If you compare the supplementary material (companions) in the two editions, the RM2/RMC material is kind of like a kit from which you can assemble a system. Whereas RMSS/RMFRP has companions with clear unified themes, and for the most part you can adopt them without issues. (I might except Fire & Ice from that, since it changes the elemental theory quite a bit. It's cool but will have more impact if you choose to adopt it.)