For an audition for a teacher, I would think that the teacher would be looking for variety, depth of musical expression and a reasonable technique - for this reason, it would be best to play whatever would show your best skills as a musician. Not knowing how you play these pieces, I would think that the Schubert Impromptu would give more variation to your performance, than that of the second movement of the Mozart, having already played the first. However, if you feel that you can play the Mozart much more convincingly than the Schubert, then this would be the best performance. Remember if someone is going to teach you, they want to be able to see what aspects can help you with: a teacher would be far more inclined to teach someone who can play musically than someone with an amazing technique but little expression.