The secondary slang meaning of "mistress" is so prevalent, that you'd say neither.You say, "And here are our emcees, Jane and Sally!" or "Here is our emcee, Jane!" "emcee" is "MC", which stands for...
The mistress of ceremonies is a woman who is running the program. She is the person standing in front of the group introducing the various speakers. She recognizes the various people asking questions...
The female version of Master is Mistress, thus it would be Mistress of Ceremony. "Mistress of Ceremonies" would be the plural, and would only be used if you were the Mistress of Ceremony for multiple...