It depends, I think, on how the movie is made. Some, like the second Potter movie, is not much more than an illustration to the book. It doesn't make much sense if you haven't read the book because the plots aren't developed enough, it's just a big mess of scenes piled up. So in those cases I prefer to read the book first. A badly made movie can make me
want to read the book sometimes! But if the movie is well scripted it doesn't matter to me which I read/see first. I just see it as different retellings of the same story, two different artforms. I'm one of those that (usually) don't care about "spoilers", because I tend to be more interested in the style and moods than in the actual plot. It's true - I almost never remember the ending of anything, I just remember the language and colours and stuff like that.
Opera, now that's a different matter. Nowadays I always read up on the opera before I go see it. (Sitting through
The Marriage of Figaro without any prior knowledge of the plot is a very bad idea. Just FYI.
)