I think they sort of exist on a sliding scale of how much you know about the fight and how much you'll be expected to carry.

No fight knowledge, plus expecting a lot of deaths? Go RDM.
Decent fight knowledge, maybe need to do occasional rez? Go SMN.
Excellent fight knowledge, party has this thing down to a tee? Go BLM.

That said, I enjoy SMN most, I don't tend to ever move to BLM stage just because I'm not a huge fan of it. Still fun now and again though.