Whatever song you select, and Marian's songs are a good place to start, make sure the song can be used for a while. You can't go off and find a new song for every audition. You will find that you don't have time to work on text and/or subtext. Choose a song you can do for years.
Auditioning is more than hitting the notes. Who cares if you are pitch perfect if there is nothing else to make you memorable? Sarah can be a dull role unless you can give it a push. Make her sexy, and for that matter, add some heat to Til There Was You. Nothing overt, but sneak a little naughty yearning into it.
"Til there Was You" is a better audition song than "My White Knight" because it is the basic 32 bar form and, it builds nicely. My White Knight is a good song, make no mistake, but only if you are asked to sing a second song. It has several false climaxes, and it might be tough getting their attention back after you hit an emotional high point and then have to pull back.
Look at other classic leading ladies too. Brigadoon comes to mind right off the bat.
"If my life weren't funny, it would just be true. And that would be unacceptable."
--Carrie Fisher