Has your director said anything about singing songs from the show? If he or she specifically said to sing something from the show, sing "I'll Know." If he didn't, do something else...I generally think "My White Knight," from Music Man, is a better audition song for Sarah than "Good Night My Someone" (provided it's not overdone to death where you are). In terms of lyric, "My White Knight" is very similar to "I'll Know," and the range it showcases is higher than that of "Good Night My Someone" ... Sarah needs a strong G5-Ab5, which is within "My White Knight" range.
I played Sarah over the summer, so if you have any additional questions about the part, shoot me a PM