In My Antonia, who learns more through their friendship, Jim or Antonia?

Though Jim is the protagonist of the story, it is difficult to ascertain who would have the greatest impact on the other. It is easier to see the impact Antonia has on Jim as Jim is the protagonist of the story, and we have his point of view. My opinion is that Antonia has a greater impact on Jim in a sense. Jim is privileged, while Antonia lives a very hard life. She is not well off and is deceived by a lover in the book; she is then left alone and with her child. There is a sense that Antonia is always surviving, and while Jim is still important to her as the years pass, she survives alone until she remarries. Meanwhile, Jim always thinks of Antonia as another route he could have taken in his life, and fate just did not turn that direction for him. Cather writes,

Whatever we had missed, we possessed together the precious, the incommunicable past.

This quote is important because it highlights one of the major themes of the story—we carry the past with us, along with the question of what might have been.

I think most people would say Jim learns the most, since Jim is the narrator and it is really his story that we experience in the novel. Antonia comes to represent for Jim a kind of idealized version of some great truth about life. He objectifies her, in the sense that he has trouble sorting out his feelings for her—she is at once someone he deeply identifies with yet also fundamentally different; he wants to protect her, and as they grow up he has some confused half-formed romantic ideas about Antonia. He senses something in her—he feels he understands her in a deep way. The poetry of the book lies in how it puts this empathic knowledge of Antonia on the same footing as the practical knowledge of life on the plains.
Antonia comes to learn about the nature of death and about self-sacrifice and her own immense emotional strength. Although she is betrayed by one lover, she finds it possible to marry and have a family. Her journey is much harder than Jim’s, who has a life of privilege at college (and has the common decency to know it). It is an open question whether Jim ever fully understands her transformation or what her struggles have been.
One of the reasons the book is so compelling is that these questions can’t be answered. When Jim returns at the end of the book and visits the adult Antonia on her farm, it is both cathartic, in the sense that we are glad to see these people together, and bittersweet, in that each has chosen a different path in life.