In most cases the FM are games born out of the whiny japanese fanbase that is vastly superior to the original. In this case though BBSFM has less added stuff than any FM before and basically they got 2 mirage arena boss battles, a new mirage arena area, and a new secret ending. Nothing else worth mentioning.
So I'd say just go and get the normal BBS and save yourself the money then youtube the new secret ending when it comes out.
Official Roxas of the Kingdom Hearts: Birth By Sleep Board ^_^