Better to start new character and new class or use one character only

[Vandril]

Though it may seem like it will take longer to level secondary+ classes due to not having quests to gain exp from, that's not rally the case. You gain a +50% exp modifier on all subsequent classes and really quests take quite a lot of time for the exp that they give. You just odn't notice it so much when doing them the first time because you're interested in the quest itself.

Sometime during Heavensward it became a 100% EXP bonus when your lower level classes are below level 50. From 50-60 it's still 50%.