Dipyaman Roy

We can't say which is more difficult? Piano seems to be easy because you have to press keys to produce the note, whereas in guitar you have to fret the note and pluck the string to produce a note. Once you start learning guitar or piano you will find difficulties playing. Both has it's own limitations and characters.

I think producing a piece of music itself is anyway a big deal, since you have to work hard on your instrument to understand it then playing some tune which resembles a specific story is difficult, no matter its Guitar or Piano

Guitar is the most versatile instrument ever, it can produce both polyphonic harmonies (chords) and monophonic (single note melodies) like piano. But the size of the instrument made it easier to carry and it became a very important part of gypsies who travelled from spain to middle East or to Latin America which made it the most important instrument for a band, much later it went electric and many modifications were done gradually so that it can be played in huge arenas. 

Apart from all these the design of guitar is made such a way it stays so close to your heart that you fell in love with the sound it produces. 

Listen: Listen to music that has harmonic changes, like Western classical/Jazz. It's the best way to develop your ear. 

Practice: Practice guitar as much as possible, when I say practice, I mean useful practice. It's not like holding the guitar and checking your phone. If you can practice for only 10 mins, do that properly and in a continuous manner.

What to practice? 

If you are very beginner, practice basic scales, chords, finger exercise. 

If you think you are not befinner try playing things you have never played before. Like if you only play rock try some other genres. 

Also play a lot of rhythm guitar. All guitar players like to play guitar solos but doesn't like to play rhythm, but believe me practicing rhythm guitar will make your lead playing a lot better. 

Always try to record when you play and listen to what you have played, analyse and try to develop a critic inside you to criticize your own music.

There are examples of great guitar players who doesn't know much about theories but they are genius. Also there are guitar players like Steve Vai, John Petrucci and many more who has a very good understanding of music theory and they apply them in their music. 

In my personal opinion, music theory is kind of a language and tool which helps us understand or communicate in a better way. Some people say that they like to play from their heart, but you need to develop your inner musician to be able to play from your heart,  music theory will save you in situations like improvising with a chord progression that you have never played before. 

Learn music theory, it's necessary for those who are not born as a musician.