I think RMFRP is organized a little better, but it still has the rules first with the details of the races, classes, etc in the back. RMFRP does have everything you need in a single volume (rules, attack tables, spell lists) but only for a smaller set of races, professions, etc. You get the rest in Character Law. RMSS doesn't have the attack tables (those are in Arms Law) or spell lists (in Spell Law) but it does put a larger set of races and professions in one volume.
Ultimately, I don't think they are that different, and the rules are almost entirely the same.