I've enjoyed the analysis of Willoughby's confession--its sincerity or lack thereof, its attempts at passing blame to others, its meandering narrative; I've seen it compared to Brandon's disclosure to Elinor of the saga of the Elizas (I don't find the Colonel's history a confession but a confidence) but I don't recall any comparisons of Willoughby's confession to Marianne's. So here goes my attempt at one. Because W's has been so extensively discussed, I'll concentrate on Marianne's.

First, Marianne's confession is only a page long, compared to Willoughby's many page intrusion on Elinor's time. She credit's her illness with insights into her behavior & attitudes:
"My illness has made me think--It has given me leisure and calmness for serious recollection. Long before I was enough recovered to talk, I was perfectly able to reflect." In some ways I see this long febrile illness of hers as a sort of crucible or purgatory that burned away her faults--or at least made her aware that they were faults. "I saw that my own feelings had prepared my sufferings, and that my want of fortitude under them had almost led me to the grave."
She takes full responsibility for her behavior: "My illness, I well knew, had been entirely brought on by myself by such negligence of my own health, as I felt even at the time to be wrong. Had I died--it would have been self-destruction."
She was eager for recovery "to have time for atonement to my God, and to you all,"
She is sorry, ashamed of her past behavior for its imprudence but also for its arrogance:"I cannot express my own abhorrence of myself. Whenever I looked towards the past, I saw some duty neglected, or some failing indulged."
The last part of her confession echoes some sentiments in the Act of Contrition & Confeteor (these are Catholic prayers & I don't know if there is anything comparable in the CofE) The form of Marianne's recital follows the form of the prayers--acknowledging wrongdoing, professing sorrow, taking responsibility and resolving to improve and avoid sin in future.

Marianne's expression of sorrow contains all the elements of true confession put simply and elegantly. She doesn't ask for forgiveness, she knows she is forgiven & loved;

Willoughby's story demands attention to persuade to forgiveness, but bypasses the elements of taking responsibility, true sorrow for the wrongdoing, and resolving to improve--in fact he pretty much says he won't improve.