Guitar chords which is better

Very George Harrison! Personally I prefer the G to Gdim. I think the G6 a little 'light' if going to Gm. However both sets work. It depends on the rest of the song and it's overall 'flavour'. Ain't music fun!!

As Steve said,"It depends on the rest of the song." You can do either, or anything you want, but they both have different harmonic function.

Gdim and Gm point in different directions. Diminished chords are very tense and will be most useful when resolved. Gdim like all diminished and dim7 chords can be resolved up a half step, a m2, or down a whole step, a M2 (up one fret or down two).

Diminished chords are most useful because they are very ambiguous. A Cdim is spelled C-Eb-Gb and add a Bbb (B double flat, enharmonic equivalent of A) for a seventh, then compare it to an Ebdim7: Eb-Gb-Bbb-Dbb. A little mental adjustment and, bam!, the same chord. Any note in a dim7 chord can be the root. Depending on the style of music any triad can have a seventh added. So since dim7 chords exist in eight keys depending on usage, dim and dim 7 can move to any chord root a half step up or whole step down from any note said diminished chord. Listen to anything by Jobim to get the sound.

Alternately, the Gm will either lead down a fifth for a very smooth sound, up or down a third/sixth for a jarring change, or up or down by whole/half step for a less jarring change.

The real difference between Gm and Gdim is that the Gdim really needs to resolve, but the Gm could make up the rest of the song. It would be a very boring and sad song.

The G6 is an enharmonic equivalent to Em7. It is most likely an addition made by a transcriber to keep a song properly in the key of G while accounting for an e in the melody.