Cold star in neutral > Dark Hole in neutral. Outside of a few characters, cold star has better applications than Dark Hole. Cold star can be used defensively because it comes out faster than Dark Hole and ammy being in the air catches bad air dashing and allows you to combo. Dark hole comes out too slow to be better than cold star. You almost have to use it pre-emptively in neutral, and we know what can happen if you call an assist out wrong (not saying cold star has the same problem but the start up itself helps you not get blown up on reaction).
Phantasy is 100% right about them both being subpar compared to missiles, beam, jam session, vajra, etc. But if you gotta make me choose, cold star is just the better overall assist. Dark hole has good uses though, its not useless like people here play it out to be.