Definitely Luna! Lily is good too but it can sound trendy sometimes. I think Lily is best with longer, classic middle names. Luna is mythological, celestial, and a bit mysterious but still simple and easy to manage. It's very usable while still giving off an interesting impression.
Definitely Lily. It's classic, adorable and very beautiful. I never cared for Luna. It just reminds me of words such as 'loon, lunatic, Looney bin and so on.' Also, I just feel that Luna fits better as a pets or character name rather than someone's actual name.