Advantages to doing it all on one character:
  • You don't need to repeat the Main Scenario.
  • cross-class skills can be easier to get, if the class you'd get them from is one whose job you're leveling up anyway. (Though it sounds like this advantage might be going away next month, depending on whether some of the changes I've heard about for StormBlood affect only the new SB jobs or the previous ones as well.)
  • You can earn or make gear for all your classes/jobs without needing to worry about what you can or can't transfer to other characters.

Advantages to spreading it between multiple characters:
  • You get to repeat the Main Scenario. (Yeah, that's on both lists. Some people like getting to repeat it. Others don't.)
  • You have a lot more room to store your gear, and won't be constantly having to choose what to throw away every time you get a new item. (You can even have room to keep a few glamour sets on hand.)
  • When end-game content has weekly caps or lockouts, you get to meet the cap for each job if they're spread between different characters, rather than having to choose which to gear up if your jobs are all on one character. (This applies only if you play enough to actually meet the weekly caps multiple times over, though.)