There are a few differences between Henry James's 1880 Washington Square and William Wyler's 1949 The Heiress, but for the most part the movie stays true to the book. The one difference that alters the story dramatically is the ending.
Washington Square ends with Catherine at her house. An older, balding Morris, who had left her out to dry, returns to her door some years later and asks her to take him back. Catherine refuses to marry him. Instead she sends him packing on his way with her dignity, self-respect, and heroine qualities intact.
In The Heiress Morris (Montgomery Clift) returns far more quickly at the end of the story, but in the film he is still very handsome. Morris begs her to take him back, and Catherine says that she will and tells him when to come back to meet her for their elopement, which Morris had avoided before. When Morris returns, he calls for Catherine, but she never answers him. She plays the same cruel joke that Morris had played on her before. Morris is left out in the cold.
These two differences do not seem that big, but in reality they create two entirely different stories. Washington Square is the story of a naïve, shy heiress who matures into a smart and strong woman. She is a heroine because of her class. She realizes that she does not need a man, and she is content the way she is. In The Heiress, however, Catherine goes from a shy, naïve heiress to a revenge-seeking woman. Catherine is never totally content with her life. She has to rely on revenge and cruelty to make herself feel better. The cruel joke that she plays on Morris is the only real thing that gives her closure, whereas in Washington Square, she is content. I think that in Washington Square Catherine is more the victor and more the heroine.