How has Candida proved that Morell is weaker than Eugene?

In the play, the title character Candida is married to James Morell, an intellectual and social reformer. They love each other but James takes her for granted. Eugene Marchbanks is a younger poet who has a crush on her. Believing it is true love, he tries to convince Candida that she would be happier with him and that she should leave James.
In act 3, when the two men argue, James tells Candida that Eugene has claimed that she belongs to him, and he says she must choose between them. This does not go over well, and she speaks harshly to James about the "belongs to" idea, suggesting they sound like they are putting her up for auction.
James, already self-conscious that he can't match Eugene's poetic way with words, realizes he has totally blown it and starts to cry. That brings out Candida's affection, but she still presses him for his "bid."
James offers her, among other things, his honesty and strength, while James offers her his weakness. When she says she picks "the weaker of the two," James initially thinks she means Eugene. But she means James because he has shown that he really needs her.