I don't think there's a strict difference, but a preface would be an introduction - perhaps comprising of a summary, whereas a preview is usually a direct excerpt of what is to come. E.g. For the general elections next year... A preface might be a summary of the overall agenda of a political party. A preview might be a specific election pledge or promise that the party is going to make. Hope that helps. :)

Preview - an inspection or viewing of something before it is bought or becomes generally known and available. Preface - an introduction to a book, typically stating its subject, scope, or aims. So, yes sometimes they can be used and mean/give off the same idea.