I remember reading somewhere an older blues guitarist (Maybe Skip James?) said that you can do more with a guitar than a piano, because when you play the guitar you have three hands instead of two. I.E. The right thumb is one hand, doing the work of the piano left hand. The right hand fingers are another, doing the work of the piano right hand. And the left hand is the third, filling in with slides, pull-offs, hammer-ons, etc.

Personally I play both guitar and banjo - neither very well. Banjo is much easier for playing melodies but guitar is better for rhythm.