A great war of literary aesthetics and associations, has been raging for decades and I, for one, was not aware of it. Turns out that science fiction purists, the sophisticates, have long detested the abbreviation of science fiction as sci-fi, SciFi or scifi referring to it as “skiffy,” relegating it to a realm of stale popcorn and sticky seats entertainment — or at least stuff that’s not done or taken as seriously as it could or should be; second-class science fiction when all science fiction should be first class.
To refer to science fiction as sci-fi marked you then as an outsider to the genre, a casual, obtuse tourist clumsily tromping about in literary gardens that you really knew nothing about, stepping on the geraniums and the genre, leaving behind candy wrappers and brightly inflamed sensibilities.
The preferred abbreviation being SF, or San Francisco to most folks and also being conflated with the abbreviation for the term speculative fiction which arose from this conflict in an attempt to rebrand the genre.
Apparently, although still ongoing, the hostilities have subsided appreciably because science fiction no longer suffers from low self-esteem, it has become a firmly established literary genre and is taken seriously now, or at least far more so than it used to be by the literati; which really gets at the heart of the matter — all along it has been a fight to valorize the genre, to have it be critically accepted as literature.
I have no issues with any of the aforementioned abbreviations for science fiction and really don’t think it’s any great matter of concern anyway, but everyone’s sensibilities in this and every other matter are different. My personal favorite actually being SciFi, it looks great on the page — but if the arbiters of science fiction say that SF is the preferred abbreviation then that’s what I’ll go with.