If you asked Joss Whedon which was real, the slayer life or the institution, he'd probably go, "Wellllll....I think you have to answer that for yourself." In other words, there is no answer, because it was written to be ambiguous. But Slayer-Buffy got 7 seasons, a spinoff, a movie and a comic book series, while institution Buffy got a single episode. Believe what you want, but Slayer-Buffy has a lot more evidence.
_________________
Everything would be better if you were in charge.
