[pkautzsch]

Depends on what your main mic system is, and how much of the guitar it picks up.
If it's an ORTF over conductor's head picking up the orchestra perfectly, but *behind* the soloist, the guitar won't project into the main pair. Then a small AB pair of spots, say 20 cm, may be the way to go.
If it's a spaced pair a few feet back in the hall, and there are maybe some orchestra mics up, a mono spot can be better.