I am loyal to Princess Luna of the night myself. If I were to join then my life would be hers. I already think of her every month, week and day/night so my loyalty is sustainable. Also, she seems like a girl I would like, if not as a lover, then a protector. Combat-wise, I'm not sure I could, but maybe I can; I mean it's not liked I killed anyone (if killing is possible in Equestria). I'm more into strategic tactics and reconnaissance, in the shadows of the night like a ninja. I would think Luna would be more harsh than Celestia, thus it would contain the discipline of being a soldier serving her. Both are great rulers though, one cannot exist without the other (even though Tia took on the role of raising both sun and moon). It just wouldn't be a Lunar force without Princess Luna.