Ghibli is more varied and more ambitious, although Pixar's excellent at what it does. Pixar's never done anything that's left me as amazed as after any of the following: Grave of the Fireflies, Princess Mononoke, Spirited Away, Whisper of the Heart, Porco Rosso, Nausicaa.
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Praise the Black Turtle, Game of the Decade Guru Champ.
(Well, I never promised creativity, did I?!)