I've got both class lvl 90(well..almost for the monk just miss the helmet) and i can tell it doesn't matter which one is the best beetween MNK and DRG.They're both pretty similar in term of damage,MNK having a strong sustained damage output while the DRG can put out a much stronger burst (the best in the game i think execpt for the BLM getting Fire III proc everytime).
They're both good for a party any high lvl content can use both of them.
Ofc,DRG can use his Disenbowel for the BRD and himself but seriously it's not like it will make a difference beetween the wipe and the victory.
I'm more of a DRG player but just in case i lvled both class and anyone who did the same will agree that both are REALLY ok for any kind of content.They do'nt need a adjustement or something.Just good players and good team comp.