I played both to 50 but haven't played either in Heavensward yet.

MNK is going to be enjoyable if you like positionals to maximize your DPS. DRG is going to be your alternative to that, but they have a pretty complex rotation currently from what I hear. I think DRG offers a bit more in raid utility as well, if that matters to you. Both can hit like freight trucks when played well.

Personally, I'm Team DRG.