Sci fi was the original, by analogy to high fi. I starting reading it back in the early 60s, when it was pronounced to rhyme with "high fi", then the coming thing in music. Then it was abbreviated to SF or sf by various purists who no doubt thought that the slang phrase was too, well, undignified, and thus was born the initialled version.
Me, I'm easy. But inside my head it's still sounded as "si fi" even when I see SF. Too late for me to change and I can can no reason to. But sf or SF is easier to type.
Isaac Asimov had an amusing distinction. Sci Fi was what you saw on TV or at the movies. SF is what you read.
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