Pearl, Amy is completely unpredictable and also a small woman, who isn't even a proper shortstacc
, where as Pearl is far more likely to stay in a stable mood, and simply tell you what she is thinking about, if something bothers her.
Where Amethyst would simply act impulsively.
Not only that but the first time with Pearl would pave a way of a extremely pleasurable and improving sex-life; due to her dependant nature, and desire to please/improve, whereas Amethyst is more likely to "just be as is, and other wise you can just go fuck your hand bro." type of person, meaning again it's unpredictable what she will do.
However I do agree that in a stable controlled situation Amethyst's lips would be more pleasurable, but Pearl is more likely to handle some technique.