Throw my vote in for "both and neither" also. I run my own settings and my own fluff, but I love to read published fluff and mine it for ideas and inspiration. So, even though I can generate my own fluff, I would still lean closer to all "fluff" books than to all mechanics since my games tend toward cooperative storytelling than "games" in the literal sense. On the other hand, I'd rather not have to generate my own mechanics and having the tools and the framework already there lets the story flow better.
So, if I had to choose one or the other...I wouldn't.