As I have said before... There is a reason they teach college level music theory on the piano. It is easy to explore extended chords with much less exasperation with the finger straining gymnastics involved. It also lays out the C scale very clearly and this helps newbes get a grasp on music theory and starting to play musical scales. If you take college level theory you required to have piano competency, or you have to be currently taking lessons on getting there. The down side is that every single musical key has a diffrent fingering pattern and to become competent you will need to learn all of those diverse fingering patterns. It seem to me that chords are easier to master on a key board and scales are easier on the guitar.

Guitar is much easier to transpose music into another key since most of the fingering patterns are similar. You move a C major scale up the neck 2 frets and you are playing in the key of D major. Move up another 2 frets and you can play in the key of E major. Same pattern! This applies to all up and down the neck. So to play major scales you only need learn one pattern and move it up and down the neck. This makes playing easier but it could make understanding the underlying theory more elusive.

As far as theory... you can learn it on either insturment, though the piano probably has the advantage.