Is The Adventures of Huckleberry Finn more realistic than The Narrative of the Life of Frederick Douglass?
No, not remotely. Huck Finn is a pure work of fiction, so you find speeches people will never make and all the conventions of fiction. It is also written by a white man, who tempers his views for a buying public. It might be more powerful, but it also is as contrived as any hollywood movie. Life is autobiographical, which might have exaggerations, but is still written by a man who was an actual slave. While it was designed to make a point, it still stands as a first person account of slavery.