I'm just going to stand in the middle and say all equipment is situational. But I do feel MNKs should try to aim for Black Belt because of the 5% more haste plus Subtle Blow and Physical Defense Down is pretty good for attacking fast, giving less TP to monsters, and getting some defense to boot. Twilight Belt is given to other jobs and is one of the rewards to the Abbysea quests/storyline, which may serve as why Twilight Belt is not as great as the HNM/KSNM trials of the Black Belt.