There is no doubt that while the book and the movie share major plot points, there are some serious differences. Certain story-lines were condensed, combined, or cut for the purposes of the film. For the most part, I felt like these were good decisions, since the novel is a rich book with lots of details that are difficult to convey in a few hours onscreen. For example, the book ends with Francie and Neeley as young adults, but the movie stops while they're still children. The movie does just fine without the latter parts of the book and I think shortening the story was a prudent decision.
However, while some of the changes were true to the nature of the story, others were not. The movie definitely waters down the grittiness of poverty that the novel captures so well. Some of this can be attributed to the censorship in the 1940's. For instance, pregnant women were not allowed to appear onscreen. The notion was unthinkable in those days. Needless to say, my sisters were amazed at how thin these women of a bygone era were, even when months with child! There were other more mature themes that didn't make the cut: the complicated details of Sissy Nolan's multiple "marriages" and miscarriages (simplified and glossed over in the film), the pain of childbirth, and rape/sexual perversity. While the simplification and sometimes omission of these difficult issues certainly gave the film a level of charm, I think it came at a high price. The impact of the book was not fully present in the movie.
Even themes that did make the movie were weakened, due to the sensitivities of the Greatest Generation, its audience. Johnny Nolan's drunkenness was present, but kept off screen much of the time. More time was spent on Katie Nolan's faults than the complicated vices of her husband. The constant struggle for money wasn't as prominent. There were some allusions to Sissy's strange past, but it was never fully explored. Basically, while all the difficult questions and ugly sides of people were there, they were airbrushed to a certain extent in order to make them screen-worthy.
There was one scene of the film that definitely outdid the book, in my opinion. (It's a minor detail, so I don't think I'll be spoiling the larger plot for anyone interested in reading/watching A Tree Grows in Brooklyn!). Just after Francie and Neeley graduate, they all go out for ice creams and sodas, and a young boy comes over to the table to ask Francie to the movies. The moment is short in the book -- it's only meant to indicate that Francie is growing up and things are changing -- but it becomes a hysterical and somewhat longer ordeal in the movie. There are no clips of the scene online, but even if you don't plan to watch the whole film, watch that scene for a good laugh. It's adorable.
Book or movie? Perhaps this is a cop out answer, but I'm going to go with both. I think that while both novel and film tell the same basic story, in the end they are different tales. Betty Smith's novel is a little more beat up, with more grime and dirt and some holes in the knees of the jeans. The Nolan's poverty doesn't have the novelty it does in the movie. The morality of the characters is more complex and questionable. And following Francie and Neeley into adulthood certainly changes the message a bit. Meanwhile, the film is wonderful. I cried, and so did my parents. The acting is superb. I especially love the girl who portrays Francie, Peggy Ann Garner. She's absolutely darling. Sure, the movie isn't as gritty as the book, but it still addresses universal questions and does a great job. So my answers is "both", which I think is feasible, as the movie takes on a life of its own separate from the book.
Isn't this is a beautiful scene? This captures the predominant questions of both the novel and the film. The themes of education, truth, beauty, morality... All set in an impoverished neighborhood in Brooklyn. It's so awesome.
If you're interested in reading the book, click here. And here's where you can find out about the movie. If anyone here has read it or seen it, I would love to hear your thoughts!