While both fairy tales provide great messages, Beauty and the Beast had well developed characters, a great story line, and explored real world problems that helped better state the message. The classic fairytale shows the story of an intelligent young girl with a thirst for knowledge who saw past the ugly exterior of a beast, and into a heart of gold. Both characters managed to stay decent people and true to their heart despite the terrible treatment and abuse they faced. Belle was known for her eye-catching beauty, but this characteristic of hers paled in comparison to her brain and her heart. While the beast had issues, Belle's kind-nature helped him work through them, her presence turned him back into his beautiful self, which perfectly represents the theme of the story.
Snow White is quite similar; she is a beautiful princess with a kind heart, who see's past the seven little dwarfs' odd personalities. However, having the antagonist obsessed with her beauty just adds emphasis to the wrong idea of the story. The queen is so obsessed with being the most beautiful, she attempts to murder an innocent girl. Instead of focusing on "beauty comes from within" we see more of "envy is powerful". While it is a great story and a strong message, the characters and plot are simply not as strong as Beauty and the Beast.
A stories antagonist is a great way for an author to present the obstacle, and therefore the theme of a story. Because the queen in Snow White's story was so obsessed with her looks, her envy and jealousy drove her mad. Beauty and the Beast had everyone, even Belle for a little while, thinking that the Prince was in fact, a beast. However, Belle eventually saw inside the Prince's heart, and learned about true beauty. Both stories have strong messages and great story lines, but Beauty and the Beast's advanced use of characters and villains helped develop the theme in a powerful way, which is the mark of a great story.
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I didn't have much time to write it so it may not be the best, but I hope it helped.