Piano, because it's more physical. With a guitar you're controlling your moves from below or sideways - you control the fingerboard with your arm reaching up, and you control the strings at the soundhole with your arm coming up and across.
With the piano you use your hands with the power coming from the back & shoulders down through the arms to the hands, with your strength coming up through the floor to your legs and back - plus pedaling with your feet.
A lot more of you goes into the piano and therefore into the music, than into the guitar.
Both are awesome instruments for different purposes, but I voted piano.