The guitar and the piano are two of the most popular instruments which allow multiple notes to be played simultaneously.
The acoustic quality of the two instruments are, of course, different, but that is not the focus of this question.
The design of the two instruments affects what and how music are played. Here are some differences I have in mind:
All semitones are treated equally on the guitar. Guitar chords (other than the open chords) can be moved up and down the fretboard without changing the finger shape. This also allows the player to shift the key easily. On the piano, the same chord takes different shapes in different roots due to the presence of black and white keys.
Regarding playing multiple notes at the same time, the piano is more flexible than the guitar.
The pianist use ten fingers to play the notes, while the guitarist uses only four (or five if you use the thumb), and at most six notes can be played at the same time (using a bar)
Since you use two hands on the piano, it's easier to play rhythm and melody at the same time.
On a guitar you can only play one note on a string, so the possible voicing of chords is restricted. On the piano you can pretty much play any voicing.
Strings on the guitar can be bended, for example, to create a vibrating sound. I'm not aware of an equivalent technique on the piano.
The piano has a wider pitch range than the guitar.
Do you agree? What other important differences can you think of? What are the implications of these differences, if any?