in general, a stiiffer board works better at higher speeds and in crappier/chopped up/firm snow than a softer board, and that doesn't mean a softer board is better at slower speeds or fresh snow etc
all rockered boards are somewhat more "unstable" than cambered boards, how much so depends on the type of rocker and he boards design
all snowboards work, and everything is a trade off, choose a softer board if you don't really need stabillity to charge big nountains and crappy snow (read: you ride alot of park) and deal with its shortcomings when you do, or choose a stiffer board that will be better at charging and deal with it being slightly more work to ride in a park setting