I love the haunting melody of a harp; pianos are lovely, but something about a harp just pulls at my heartstrings (..haha).
It does, of course, depend on the emotion you want to envoke.
As others have said, it depends on the piece of music. I play piano and enjoy the fact that this instrument is fairly commonly available, so from that standpoint, I treasure the piano as an instrument.
On the other hand, the beauty of the sound produced by a harp is beyond description, in my opinion. I'd love to learn how to play one!
I would have to go with the piano. A harp may be more versatile than a person thinks, but how often do we really see one? From classical to jazz to gospel to rock and roll, piano fits into almost every musical situation, and can hold its own without being lost among the other instruments. A harp often times needs to be a solo act for that very reason.
After visiting Scotland and Ireland, a person learns that the harp is more versatile than she has thought. For aesthetics it is superior--heavenly in sound and evocation of emotion. However, the piano is the instrument for all genres. Ragtime, Jazz-fusion, Jazz, Blues, Pop, Rock'n'Roll, Country, Honky-Tonk, Classical--an essential.
I find that the difference between the two instruments rests in the place I find them. I love hearing a harp at a wedding. The music is light and relaxing. However, I also find that the piano speaks to me in a more emotional way. I love the piano the way Billy Joel plays it, and the way Marc Cohn, "Walking in Memphis" and even the theme to Twilight: "Bella's Theme." I like boogie woogie, honky tonk and classical music. The piano makes me want to tap my foot, but it can also make me teary. Certainly the melody makes a big difference, and I love lyrics, too, but where the harp is sweet and mellow, the piano can take me through an variety of moods. I've always had a soft spot for the piano.
I'm not a musician, but I think I'd definitely have to go with the piano. I'm basing that opinion on the music I've heard using each instrument. It seems like the piano is a more versatile instrument. Harps seem to be more of a specialized instrument. Harps do have a nice, pleasant sound, but nothing I've heard comes close to matching what I've heard done on a piano.
I love the beautiful sound of the harp, and it is one of the most difficult instruments to master, but the piano is a far more flexible and accessible instrument. The harp's ethereal sound is virtually unrivaled by any other instrument, while the piano is an essential part of nearly any kind of music.
I, too, think it depends upon the situation. Many times, harps are only used as a solo piece; pianos, on the other hand, can be used with other instruments. That said, I really like both instruments. I do have to say that I have a new appreciation for the harp though, after watching a person play the "Earth Harp" on "America's Got Talent." Beautiful, wonderful sound.
I think it depends on the situation. The piano can be far more varied in the type of sounds that it produces. The harp does not lend itself well to a variety of musical styles. However, the harp has always held a sort of fascination for me. Perhaps it is because I know how to play the piano that I do not have the same awe when watching it played. The harp seems complex and complicated in a way the piano does not.