I definitely prefer the tyrant version of Celestia, mostly because I'd love it if the show were more subtle than it actually is. Between the two princesses, though, I've got to say Luna. She seems to have a more complex and interesting personality, whereas Celestia only seems to be the embidoment of wisdom (no matter how much the universe has to bend to suit her advice) and that of a "cool" ruler. I'd like to know more about Luna - I don't really care about Celestia.