Which is more difficult: playing piano or playing guitar?

there is a saying "piano is the most easiest instrument to learn and the hardest to master" 

there actually can't be a comparison though both instruments have their own challenges..the guitar has evolved to become more of a forefront instrument than a backing instrument also the evolution of playing the acoustic guitar more percussively has taken the notch up.

Both piano and guitar offer their unique challenges, and the difficulty of learning each instrument can vary depending on individual factors such as prior musical experience, learning style, and personal preferences. Here are some considerations for each:

Piano:

a) Coordination: Playing piano requires coordination between both hands, as each hand typically plays different parts simultaneously. This can be challenging for beginners who are not used to coordinating their hands independently.

b) Reading Music: Piano music is typically written on two staves, with the right hand playing the treble clef and the left hand playing the bass clef. Learning to read and interpret music on both staves simultaneously can be daunting for some beginners.

c) Technique: Piano technique involves proper hand position, finger placement, and control over dynamics (loudness and softness). Developing good piano technique requires consistent practice and attention to detail.

d) Visual Layout: The layout of the piano keyboard is linear and logical, with each note corresponding to a specific key. Some people find this layout easier to understand than the fretboard of a guitar.

Guitar:

a) Fretboard Knowledge: Learning to navigate the guitar fretboard and memorize chord shapes, scales, and patterns can be challenging, especially for beginners. It requires developing spatial awareness and muscle memory.

b) Chord Transitions: Guitarists often struggle with smooth and quick chord transitions, as changing between chords requires coordination and dexterity in both hands.

c) Strumming and Picking: Mastering strumming and picking techniques, as well as learning to control dynamics and rhythm, can take time and practice.

d) String Bending and Vibrato: Advanced guitar techniques such as string bending and vibrato require precise control over finger pressure and pitch, which can be challenging to develop.