Which material is better for guitar strings? Nylon or Steel?

Nylon and steel strings are NOT interchangeable, so once you've chosen a guitar, you have to stay with the type of strings the guitar was built for.

If you're trying to decide which is best, a nylon or steel stringed guitar, neither is "better". It comes down to which is most appropriate for the genre of music you expect to play. Nylon is generally used for classical, flamenco, and folk music. Steel is more appropriate for rock, blues, pop, country, etc. Of course, there's nothing preventing a person from playing any style on either type of guitar, it's more an issue of the tone.

Beginners often find that nylon strings are easier on their fingers, being both low tension and fatter than steel. This, however, shouldn't be a determining factor in choosing a guitar.

Depends on which acoustic guitar you have. The classical acoustic was built to b used with nylon strings. The narrower neck acoustic was built to be used with steel strings. Both guitar can play all types/genre of music. Many people think that the nylon string guitar is only for playing classical. Wrong belief, It can play any type of music. Watch jazz great Earl Klugh in You Tube playing his nylon string guitar in all his jazz and other genre with the classical guitar. On the same note the steel strings guitar can also play classical music just as well. It is a matter of personal choice. One word of caution. .Never place steel strings in a guitar built to be use with nylon strings. It will wreck the neck in a little time. Steel strings places a lot of pressure on the neck of a guitar. That is why steel strings acoustics have an adjusting rod built on them.

Steel strings produce a more bright-metallic sound. Nylon strings produce a more darker mellower sound. Check playing both types and find out for yourself.

Nylon strings are imperative for classical/Spanish guitars. Steel strings will damage those type of guitars.

Steel string or "regular" acoustic guitars work best with (you guessed it) steel strings. In fact stringing a steel string guitar with nylon strings wouldn't be very feasible and if you did manage it it would sound stupid.

Depends on what you want to play. Nylon strings are generally on classical guitars which have more space between the strings (that is a huge generalization and may be specific to where I live, but you get the idea). Nylon strings take less pressure though and may be easier to manipulate if your fingers are sore or you have physical issues (arthritis, blisters, etc)

Steel sounds better; nylon feels better.

But both work really well. Try both and find your own preference.

It's not a question of being "better" - you HAVE to use the ones that your guitar was built to take.