I've been tinkering around with both, here's what I've seen so far. Seems about the same as ShB. RDM is better than SMN at: progression, soloing overworld, hunts, soloing potd, assisting fc mates with whatever, covering for other players (people are still learning how to Sage), and preventing wipes. SMN used to be better at dps, but right now it's not.

Both have enough mobility for literally everything, so it doesn't matter if one has slightly more mobility than the other.

I love SMN, it's an absolute pleasure to play. But I can't honestly think of a single place where it outperforms RDM. Maybe when cleave is needed? Right now I plan on maining SMN in EW regardless, it's just too much fun to walk away from.