Dark Knight is a solid choice at the moment and is one of the best Black units to have in the end-game levels. He's a Dark Fighter version of Sybilla, except he deals very high physical damage instead of very high magical damage. Of course, it does require awakening him and changing his skill for him to be very effective and useful. Since you're new, he won't be of use except for being a bruiser unit until you reach a level where you can awaken him and get his Skill AW. It is worth the time and effort since he's been proven to be very valuable by Japanese players.

Cynthia is unknown at the moment. Until she gets her awakening and Skill AW, then no one knows how good she is. She has the flaw where deploying her will increase the cost of every melee unit by 5 just for a 10% increase in ATK/DEF for said melee units.

For now, I would suggest holding onto the Rainbow Crystals until you are sure about getting either one of those units. I would even suggest waiting until Cynthia's awakening is revealed so her capabilities are more known.