I definitely prefer to read autobiographies/memoirs written by the actual person. I'm not a huge fan of first person fiction, but I love it in the autobiography/memoir style. Personal favorites: All Over But the Shoutin' by Rick Bragg, House Rules by Rachel Sontag, Divided Minds by Pamela Spiro Wagner, Crossbearer by Joe Eszterhas, and both volumes of Persepolis by Marjane Satrapi.
One of things that most people consider a drawback in memoirs is the fact that the narrator is not necessarily reliable. I actually love this part of memoirs. It isn't a heavily researched, detail driven day by day description of the individual's life. Memoirs typically cover one aspect of the author's life (war, alcoholism, an eating disorder, domestic violence) and I really enjoy trying to read between the lines to see how reliable the author is. Do they skew everything in their favor? Do they always paint themselves in a positive light? I don't mind that the author may not be as reliable, and I enjoy something that is less fact/research-driven and more story-driven.