NSA, in my mind, is the "player friendly" association. It allows you to have up to 12 players in an elimination game. You can roster bat in pool play (usually) and during bracket play you can STILL have 12 people in the game with 9 on the field, a DH & up to 2 EP's. If you roll deep with a roster full of talent, then NSA allows lots of playing time for the entire roster.
There are a few other rules differences as well. Study up so you aren't caught off-guard.