All 3 mid casters are solid and are more than capable in good hands.
Generally speaking, I believe the min/max crowd prefers RM>BD>SM in RvR and SM>RM>BD for PvE, but I'm a firm believer of play whatever makes you happy.
Most guilds will happily take a sm who is good at what they do and enjoys learning their class and striving to improve over a try-hard RM who only shows up for scheduled rvr time and hates their life because the 'meta' forces them to play a class they dislike.