If you like playing Dragoon, I would suggest keep playing it (regardless if you intend to raid or not) until 70. If you're not too hyped about continuing as a Dragoon and REALLY want to either play SAM or RDM, I would suggest SAM if you're used to being in melee, RDM if you want to experience being a caster with some melee abilities. Either way getting a job to 70 first and foremost will give you an Armory bonus that will ease your leveling experience. Ultimately I would give both of them a try and see which one you enjoy more.