It would not be a SS to say "I have been praying about this for a few days...but I just don't think I can perform this song in a way that meets your vision for it. As you know, I am used to singing it in a more classical style and I think that the contemporary interpretation you are looking for is intriging, but not best supported by me. Again, I am sorry, but our voices are just not blending on this so I am going to have to step down."
You are not saying your way is right, but that you can't do what she wants to accomplish.