There's differences, but not huge ones, and I've never had much problem adapting RM2 stuff to RMSS or vice versa.
Agreed, using elements of one in another has never been much of a problem for me.
I find both systems to work, and that the major variation between them is "Which one are you most comfortable with?"
Well, that can be said of all things. Except there are
those people. You know the ones, they specifically do things that are outside their comfort zone. Like, like, like ADVENTURERS! Crazy people, I tell ya. Ever last one of 'um.