It not that hard to get exp on an alt job. There is a lot of exp bonuses that stack: the armory bonus, FC or Scroll bonus, food bonus etc. I've leveled both alt jobs and an alt character and leveling an alt job on a character that already has completed the MSQ/unlocks etc is far faster in my experience than leveling a alt character. Just make sure that you are taking advantage of all the different ways to boost your exp.