I prefer the preface to the introduction. The preface puts forward a brief summary of the chapters and says what the book is going to be about. Writing a preface is like writing a grant proposal or plan for the book. The nice thing about it is that you are writing the book while planning to write it at the same time.
Introductions are typically longer, and include more contextual and theoretical background. What I like to do is to make the first chapter do double duty: be introductory in some way, but also present substantive, non-introductory points. For example, in Apocryphal Lorca I used the first chapter to talk about Lorca himself, rather than his American imitators. I wasn't introducing the book, but writing another kind of essay that prepared the ground for the rest of the book.
I'm not saying this is the right way to do it. Your project--or your own personal style of doing things--might require an introduction rather than preface--or both. If you have both you have to keep their functions rigorously separated, which I've always found difficult.