An autobiography covers a whole life, usually in order, from early memories to the present. A memoir takes one slice of it, such as a career, an illness, a marriage or a year abroad, and examines it closely. Both are written in the first person and both are true, but they ask different things of the writer.
If your answer to "what is this book about?" is "my life," you want an autobiography. If your answer is "what happened when I lost the business" or "how I learned to walk again," a memoir will serve you better, and it is often the shorter, cheaper and more focused choice.
We will tell you honestly which one fits during your first call. Some clients start with an autobiography in mind and leave with a tightly shaped memoir. Others realize they need the full sweep, because the early chapters explain the later ones. Either way, the decision comes before the writing, not after.