I love both songs and I think they function differently and usefully, and so therefor I also don't think they can really replace each other without losing something. And if I had to pick I think i'd pick Mein Herr because it sets up the tone of the people and place Cliff has entered *much* more effectively, and introduces Sally's specific characters and perspective more artfully and more usefully for the show. It is more worth us seeing her sexy but self-serving and narrowminded attitude toward life and relationships etc than to hear a generalized and fun song about being naughty without her mother knowing.
I completely disagree with that, and yet I understand your perspective. I guess it depends on what we each think it's more important to know about Sally, and how we're supposed to feel about her. Somewhat complicated by the fact that, of course, Sally is not technically supposed to be singing about herself in either song :-)