For me, saying the The Adventures of Huckleberry Finn argues more strongly or better against slavery than the Narrative of the Life of Fredrick Douglasis to say that life argues better than speech.
Douglas presents the reader with a true story directly demonstrating some of the many negative sides of slavery. Twain makes no direct commentary on the instution of slavery, but rather presents a picture of life that includes slavery and absurdity, sometimes in combination.
One work is polemical, the other artistic.
I do not think it argues better, but I think it argues differently. Douglass is very persuasive because of his matter of fact method of describing slavery and his honest divulgence of his own thoughts and feelings in beautiful language. Twain is more clever, creating a satire of racism.
I think another way to think of it may be perhaps that Huck Finn is a fictional story, that has many entertaining and adventurous events in it. The reader gets sucked in to the story and enjoys themselves while reading it, and before they know it, they may be attached to Huck and Jim and what happens to them. "The Narrative of the Life of Frederick Douglass" is more serious, more straightforward, and while it is one of the most powerful pieces of abolitionist material out there, Huck Finn may reach a larger audience because the reader is looking for an entertaining story.
Huck gives a first person account of his feelings toward Jim and how he isn't just property, but a person who thinks, feels, and dreams. Huck has only known one way in the south: white people live free and they own black people. It's never been any other way for him, which makes his change of opinion and position that much more powerful. He states that if setting Jim go free will land him in Hell, then he'll just go to Hell. That's a powerful statement.
Well, to be honest, I don't think it does. I think Douglass was influential as an abolitionist for a reason, or several reasons.
But…if I had to make this argument, I'd say it is superior because of the extended relationship between Jim and Huck, and, specifically, because of the way Huck changes positions throughout the book. He's so charming as a viewpoint character that he carries the reader along with him. That means that when he sees Jim as inferior, the reader starts there. When Jim is humanized through his care for his children, the reader cares more about Jim…and all Blacks are more made more human. And, when Huck comes around to supporting freedom, the reader is carried along.