Finding what's opposite is much easier than finding what is the same between these two titles. For one thing, "The Man Who Was Almost a Man" is a short story (the projected novel version was never completed) written by a (Southern-born) Northern African-American in the 1930s while Huckleberry Finn is a novel written by a Southern man in the 1880s. This makes the works vastly different at the outset. The authorial voice and experience of the first (Wright) are based on internal vision and experiences while the voice and experience of the second (Twain) are based on external observation and perception. These factors are entirely opposite.
One similarity is that the protagonist of each is a boy in the South, though this is a superficial similarity at best, especially since Dave is a teenager while Huck is still a boy. Another similarity is that each boy has in some way a rebellious spirit and a desire to go against the expectations of society and make a man of himself in his own way. This is a more substantial similarity between the two. Another similarity is that each boy is set upon a long journey of discovery by circumstances: Dave chances to see a passing train while he is thinking of the consequences of his mistake with the gun, then spontaneously escapes with the train to become a man; Huck deliberately grapples a raft to escape down river in his efforts to act like a man.
The last similarity, that of the long journey, points out another difference. While Dave's journey is the result of a spontaneous reaction, his journey ends the plot of the short story. In contrast, since Huck's journey is the result of a deliberate plan in response to circumstances, his journey comprises the plot of the novel's story. The themes of the two journeys have some similarities. Both address the coming of age theme. Both address the theme of racial oppression. Both address the theme of punishment. Both address the theme of escape from hardship.
One night we catched a little section of a lumber raft—nice pine planks. It was twelve foot wide and about fifteen or sixteen foot long, and the top stood above water six or seven inches—a solid, level floor. (Huckleberry Finn)
Then the train thundered past, .... He gripped the gun tightly; then he jerked his hand out of his pocket. ... Ahm ridin yuh ternight, so hep me Gawd! ... then he grabbed, pulled atop of a car, and lay flat. He felt his pocket; the gun was still there. Ahead the long rails ... stretching away, away to somewhere, somewhere where he could be a man ... ("The Man Who Was Almost a Man")