My brother plays the guitar and he thinks that guitar playing will give you worse calluses then if you play the harp.
I think that the harp would give you worse. But I don't know.
WHICH WOULD GIVE YOU WORSE CALLUSES?!
My brother plays guitar and his strings compare to my harp strings are thicker then harp so I would have to say guitar must be your answer to your question but harpist use both hands more then guitar which may offer more....