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It's kinda like the chicken and egg and Catch 22 at the same time.
Whether it's a movie or a book, I like to go in blind. Or as close to blind as possible after determining that it's something I'd like to check out. Meaning I avoid reading reviews and such until perhaps after. Now the only way I can think of to get a feel for whether I want to start with the book or the movie is to investigate the matter beforehand, which seems pretty much impossible without ruining the going in blind part.
I'd say, as a weak rule of thumb, chances for the book being better usually increases if the movie is heavily based on story or ideas as opposed to mood, acting performances and visuals(can't stand reading descriptive fiction), but it's not something I think really works as any kind of general guideline at all.
For instance with Picnic at Hanging Rock (1975) there's IMO not much of interest going on(I imagine for a book either) except for the plot twist itself, which I happened to find quite fascinating, but more as a concept or an idea than in the way it was presented in the movie specifically. Which got me interested in the book. But who knows whether I would have liked the book better or not if I hadn't seen the movie first. Maybe I would have thought nothing of it and ended up avoiding a movie that, despite my criticism, I quite liked?