This is a complex question. The narrator makes clear that Harlem is a place of darkness, with temptations for any child brought up there, and he notes that the same temptations still exist for the students he teaches. But Sonny's brother also remembers a support system when "old folks were talking after the big Sunday dinner" (444) and Sonny knows there is darkness and joy when he says, "All that hatred down there, all that hatred and misery and love" (458).
Sonny's brother has managed to stay in Harlem and become a "solid citizen" with a teaching career and a family. He has escaped the temptations of Harlem. But do you see anything lacking in him? Does he have any soaring joy in his life? He is not always the most sympathetic figure in the story. His wife is more comfortable talking to Sonny than he is, and there he is often lacking in empathy. He had written his brother out of his life. He has succeeded in not getting caught up in a life of drugs and crime by holding himself apart, but this makes him a somewhat rigid figure. This is the tradeoff for the narrator.
Sonny, on the other hand, has succumbed to the temptations but in the final scene in the story, the reader can see that he has great depth and that there is great joy as well. This, too, is a tradeoff, isn't it? In order for Sonny to "fly," he has endured great sorrow, and he talks about a woman singing the street to make his point, saying, "...it struck me all of a sudden how much suffering she must have had to go through - to sing like that" (456).
As for Sonny's biggest vice, it is clearly his drug addiction, likely an inherited addiction, since we are told that the father had a drinking problem. If Sonny had been raised on Long Island, would he have become a heroin addict? Maybe not, but he probably would have become an alcoholic.