Q. What would you trust more, in seeking to learn about a person: their biography or their memoir/autobiography?
A. I think this is quite dependent on the person. In the case of a person who is remarkably egocentric/self-absorbed, I would prefer to read a biography because it would likely be less biased in favour of the author. On the other hand, a very humble person may downplay their achievements which would also cause a negative bias towards themselves which would make a biography a bit less biased and hence more reliable.
However, you could also argue that the biases that would come in an autobiography would be useful in conveying the true personality of the author.
I would say in both cases it would depend highly on whose biography/autobiography/memoir I would be reading, and of course, in an ideal case, I would prefer reading all three and comparing their similarities and differences.