I always advocate learning a dungeon as a DPS if you're going with seasoned adventurers. It's usually less of a liability. After you learn the mechanics of a fight and are comfortable is a good time to go healer. Of course if everyone is new then go as whatever you enjoy playing.

SCH is a bit more of a forgiving healer as well which you may want to dabble in. It's easier to shield the tank/party before an attack than it is to pre-cast a heal for an attack you aren't aware of/prepared for. Thanks to the fairy you also have additional heals in case your reaction times are slower from learning new attack patterns to dodge.