Isabel Allende sets up a contrast between the two characters’ experiences in a crisis. Your opinion about who is the greater hero depends on the definition of “hero” that you apply. Although the characters endure this crisis together, the differences in their biographies make comparison difficult. You could base your decision on the immediate situation into which Allende has thrust them, or you could evaluate their relative heroism based on the biographical information she provides (which is far greater for Rolf).
As a reporter, Rolf was trained to remain above the fray, but the mudslide put him to the test. This time he gets involved, which places him in a different relationship not only to Azucena but also to his personal past, including the repressed demons that he now finds he cannot, or no longer wants to, avoid: “He realized that all his exploits as a reporter, the feats that had won him such recognition and fame, were merely an attempt to keep his most ancient fears at bay.” Because the action involving Rolf extends past the mudslide and Azucena’s death, we learn more about how she affected him. She herself could never know the impact she had on Rolf, but the reader is privy to that information, which might affect your view of either of them as heroes.
Azucena, from the story’s outset, is trapped physically by the mud and entirely dependent on other people’s actions. This virtually complete absence of agency renders her an unlikely hero, for she can do very little. Others must come to her aid, and she cannot reach out to help other victims—or so it seems. While much of the story’s action covers Azucena’s ultimate demise, both her stoical endurance of suffering and her role in Rolf’s recovery show her to be morally heroic. Only once do we hear her ask for help, when she begs Rolf not to leave her alone. After a long night, she comforts Rolf: “Don’t cry. I don’t hurt any more. I’m fine.” In the end (although neither of them intended to rescue the other), emotionally and spiritually, they play mutual redeemer roles.
Because Rolf uses his own stories and those of Eva Luna to comfort the dying girl, Allende raises the question of the redemptive power of narrative. What does it mean to "save" someone? No one manages to rescue Azucena, but Allende suggests they did not fail. All their efforts were heroic.