Creating a different character vs lvling alternate classes on same character?

[EveK]

I recommend sticking with the same character.

Although you won't have the EXP from the main scenario quests, there's no shortage of EXP from Hunting Log, roulettes, etc.

You should be getting a variety of gear along the way for additional classes, gear you couldn't trade to additional characters even if you wanted to do the same main scenario quests over again. And as you point out, having other classes at sufficient levels unlocks more jobs.