The tuning that you will use is really dependent on the style of music you are playing. If you are playing more lyrical tunes such as minstrel music, then the standard G tuning (gDGBD) is more than appropriate.
However, for older music, such as Appalachian fiddle tunes, old-time ballads and the like, it may be more appropriate to play in G Modial (gDGCD). This tuning is also known as sawmill or mountain minor. This is a good key to play solo, but with a guitar player is familiar with the style and is good at "feeling" chords changes, this is also a great key to have accompaniment.
The difficulty that guitar players have is playing in the key of G minor, which is not a "friendly" key for guitars, and many guitarists find it difficult to find where the chord changes are. A capo that allows you to switch into A minor will make it much easier for them.
Some musicians that play in this key are David Holt, Dock Boggs, and Gillian Welch.