I like Piper, it's a gp of mine. I knew the cutest little 7-yr-old Piper, and good associations always help.
Harper on the other hand, I can't stand at all. The harp- sound at the start of the name is so incredibly unattractive to my ears- it makes me think both of a nosy busybody harping on about something and also of a harpy (
http://en.wikipedia.org/wiki/Harpy). Neither of which is something that I'd want to be thinking about when I hear someone's name.