Which is better a 6 String Guitar or a 12 String Guitar ?

A 12 String Guitar has 6 courses of strings and there are two strings in each course tuned at an octave apart(see the image below). Since the tension on the body of a 12 String guitar is immense, it tends to break very soon in a matter of 2-3 years. Many of the guitarists tuned this octave lower than a regular 6 String Guitar to reduce the tension, but some who didn't want to compromise on the tone wanted to keep the original tuning.12 String Acoustic Guitars are more popular than electric ones.

A 12 String Guitar became popular amongst many rockers because of it's rich and deep ringing near orchestral sound. But it was always used as a secondary instrument to play rhythm intensive songs or accompaniment roles. The primary guitar will always be a 6 String Guitar. Because on a 12 String Guitar it was very difficult to play bass runs or riffs and bending and other techniques involving individual notes. To overcome this issue many rockers preferred to take out a string each from the bass courses and kept the treble string courses as it is. In order to switch quickly between a 6 String Guitar and a 12 String Guitar, rockers began to use double necked guitars, one having 12 Strings and the other a regular one.

Some of the famous guitarists who popularized this guitar were Slash from Guns n' Roses, Jimmy Page of Led Zeppelin and Peter Buck of R.E.M.