It turns out that the Masterclass Lady herself, Rosanne Simunovic of MasterclassLady.com, likes BOTH Siobhan Magnus and Melanie Amaro.
First, Masterclass Lady in Season 9 said of Siobhan:
"Here at MasterclassLady.com, we are not confused by brilliance. We applaud it and cherish it, feeling privileged to discover a rare and exciting talent such as Siobhan Magnus."
Also, "... we can celebrate the birth of a new and exciting star."
Also, "You are an extraordinary artist and extraordinary people will come knocking at your door post-American Idol."
Also, "... thousands upon thousands of fans love and treasure your extraordinary artistry."
Then, Masterclass Lady said of Melanie Amaro's audition:
"Quite frankly, I have never heard anything quite like this in any audition in any show. This young lady is superbly gifted and trained. She has an extraordinary vocal gift but beneath it all is a skilled vocal support system."
Also, "... note the seamless phrasing, breathing skills and circular mouth on her impeccably sustained vowels. It does not get any better than this."
But then, who is more talented, Siobhan or Melanie? It's Siobhan, because of her versatility. Melanie has a tendency to use the same vocal techniques for every song, while Siobhan can drastically change her approach from song to song depending on how she feels. She can do this because she has the ability to do this. She has more ability than Melanie, therefore she is more talented than Melanie.