Which do you prefer? Biographies written about someone? Or Autobiographies written by the actual person (and/or ghost-writer)?
It depends upon the person in question. When it comes to historical figures, I prefer a biography, especially one that puts them in their historical context. I like seeing how much someone was a product of their times or a rebel against their times. Thus, I’d rather read a biography of Voltaire rather than the autobiography he produced.
When it comes to actors and other artists I admire, though, I prefer their autobiography. I recently finished William Shatner’s latest autobiography, Up Till Now, which I quite enjoyed. Since I already live in their historical context, I’d rather hear their story from their own perspective. Artists tend to have very strong points of view, and that’s part of the reason I’m reading their autobiography, isn’t it?
The Literary Omnivore 
You make a good point about autobiographies. In the case of a well-known person/celebrity, it can be refreshing to hear the story from their point of view.
Especially since the modern cult of celebrity can involve so many lies and rumors.
I would actually prefer autobiographies if it was about historical people because, more often than not, those figures have been demonised by their society. Take Oscar Wilde for example. His rant, De Produndis, can be taken as his autobiography but it would differ completely from biographies about him.
Oh, wait – unless you meant a modern biography and not one from a peer in the same era?
I mean modern, especially concerning queer historical figures in repressive times. I’d rather read about their lives in full!