For recording solo acoustic guitar just about my favorite choice is a
pair of miniature omnis very close to the instrument, and spaced just a
few inches apart. Eliminates much of any poor room reflections and
gives a strong yet natural sound with minimal proximity boost and no
weird twangy phase problems. Like Scott, I don't care for the
artificial sound from wide spacing.

I've occasionally experimented with things like clamps onto the guitar
body to hold the mics' positions, or, a real mad-scientist approach --
lavalier mics alligator clipped on either side of my glasses! This one
really attracts attention, seems Doc Brown might have used something
similar.... g

Anyway, it's fun for singing/playing at the same time -- the vocal is
perfectly centered in the stereo field, and the guitar level and tone
can be controlled easily by just moving your head around and
closer/farther to the instrument. Not as good a guitar tone as close
in front of the soundboard, though.

BTW, any recommendations of reasonably-priced miniature omnis --
Countryman, Shure, Audix, whatever ???

Steve