Wellll out of those choices
I like NIN but not MNK too much. With SMN/SCH you get the two roles in one deal, one healer and one DPS. I don't really love SMN but SCH is a lot of fun to me. If you like pure power..then... actually, with the buffs to SMN I understand their power has gotten very large. With their AOE bursts they can do some serious damage.

NIN hits fast, with Huton up they have the fastest attack speed.


(You can always give bard a try )

Even though you don't have much time to play I suggest trying everything out until about level thirty or so- it'll give you an okay look at the basics of each class, and it shouldn't take too long. Play the one you feel you like the most.