All characters lvl at the same speed, it's how much time you wanna put into grinding that depends how fast you lvl.

Like Zeldman said, all characters will level the same way. The exp table is the same for every character. The classes with mobbing attacks tend to gain exp a little faster, due to their ability to kill multiple monsters at once, although they tend to require more potions (making funding beneficial). For PQs, assassins, clerics, CKs, fighters, and spearmen generally get invited more easily than other classes (if the party leader has their pick). So, there you have my input.