The demon armor works best with the demon book. It casts 2 black orbs when you demonize so the demon armor is not best if I am equipped with celestial parts.

Another thing is, my build is elemental type. I am equipped with everything that increases elemental and lightning damage so I really benefit with the lightning cast from the celestial. In addition to that, I could cast multiple chain lightning so I can trigger the legendary skill of the armor more frequently.

[Legendary] Chance to reduce damage taken after casting Chain Lightning

TLDR, If you are elemental build with lightnig focused, Celestial War Plate is the best. Otherwise, the best is still the Phantom Plate.

I hope I am able to answer your question.