ninja seems to have a completely different play-style from monks (they've been described as using hand-sign sequences instead of mostly positional attacks to do damage) so if you enjoy the monk play-style, i would level it

in frontlines i would say it probably doesn't matter much, but in wolves den (from my limited experience), dragoons are more burst oriented, while monks have an easier time staying on their target, so i'd say it really depends on what composition you're using for wolves den