Remember that I was drummed out of the BFA program at my college. I do, however, now teach theater and direct shows, so this is my opinion. I won't be offended if you ignore my advice.
I was always told to prepare a monologue from the show I was audtitioning for if at all possible, but what I found was if the director already had an image of the character in mind, and I didn't look like it, it became hard for that person to see me as anything else. Do you know you do this monologue better than anyone else auditioning? Have you used it and gotten cast? Then use it.
If, though, you have any qualms about using it, that lack of confidence will show through. Go in with a second monologue prepared. Spend your small allotted time to chat with the director to let him know you have a monologue from the show prepared if he should choose to see a second one. Chances are, you only get to show one, but....
Theater is so subjective. Just make sure your personality shows through. That was the lesson I never learned in college. I went in so focussed on making the character monologue work that I forget to let the director see the real me. They need to know thay can work with you and that you will fit into the ensemble.
Hope that helps.