Elinor is the guiding light of this text; without her, we'd be lost. While the story is centered on both Dashwood sisters, we definitely get more from Elinor. She's the one whose eyes we see through most often, and frankly, Austen sets us up to "get" Elinor more than anyone else, even Marianne. While the events of this plot are largely about Marianne and dealing with Marianne's troubles, the way in which we view these events are mostly shaped by Elinor's own response to them.
Marianne plays the role of the stereotypical sentimental heroine here, a figure that's thwarted (by her dumpage by Willoughby) and later reinvented by Austen. At first glance, she's the main main character, what with her youth and beauty and excitement and all, but it turns out that she's not the true heroine – her sister Elinor is. Marianne proves this right when she resolves to give up her swooning ways and be more like her staid, thoughtful, measured older sis. However, despite her problematic nature, Marianne is still at the center of the plot, and is still one of our two protagonists.