I've never heard of MLA style for fiction; I would have assumed that publishers either have a house style their copyeditors impose, or don't care what the author does as long as it's consistent.
Chicago is the underlying standard in many branches of publishing, including the ones I've been working in for years. It's a really, really handy book and good to have around. Sometimes you don't need the answer, but just an answer about capitalization, say, or whether a TV show title is italics or in quotes. Chicago's got all that, and you can't go wrong following it.
It's also easy to use—the index is excellent and the chapters are well organized, with clear outlines on the first page.
And you can almost always find the previous edition, cheap, at used book stores. I have to buy a new edition, every time one comes out, for my editing work, but if you're just using it for a reference book, the 13th or 14th is as good as the 15th.
