well maining both drg n mnk, i'll just say that :
1.DRG are WoW's fury warrior (high alpha and sustained heavy damage) with (situational) multiple gapclosers and thus are best on mobile fights while MNK are WoW's enhancement shamans (machine gun pea shooter) and thus are best on tank and spank fights.
2.difficulty wise, MNK is much more easier than DRG as their "flexible" rotations as much less forgiving than DRG's rotations in a fight where the boss are spinning or moving chaotically.
3.MNK have more party wide utility compared to DRG