Either is okay, but I prefer the sounds in the name
Melody. I've met someone named
Harmony and thought it was pleasant as a name. But
Melody seems more upbeat and joyous to me, and
Harmony seems more academic and restrained. I guess I think
Melody is a tiny bit too shiny-happy, but that doesn't really bother me much. Just enough that I would not want to be named it, not enough that I would dislike it as a name.