To me, I’d trust the autobiography more. An autobiography not only talks about a persons experiences, but also the person’s insight, thoughts, and feelings about those experiences. A biography is less personal, especially if the author is someone who does not have a personal relationship with the person they are writing about.
Also, a biography is more likely to have a plot that drives the biography since the author’s main intention is likely to sell as many copies and story-line would make it seem more appealing or interesting to people. This may lead to the person leaving parts out of the person’s life. With an autobiography, the person would tend to write what really matters to them or makes them ‘them’. Although with an autobiography the person will have a bias as it’s about themselves whereas a biography will have less bias and will try to be more objective, the reader can learn even more about the person by reading how they see themselves through their biased thoughts.
Although I believe an autobiography teaches you more about a person, reading both the autobiography and biography would be the best option. This way, you gain two perspectives of the person and can see if there are things each author has left out or was biased towards, etc.