Answer:
Both "What is the difference between X and Y?" and "What are the differences between X and Y?" are grammatical and will be understood.
Both "What is the difference between X and Y?" and "What are the differences between X and Y?" are grammatical and will be understood.According to my native-speaker intuition, "What is the difference ...?" is the normal phrasing. I would only use "What are the differences ...?" if I was already pretty sure that there was more than one difference.
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