I just responded in this same vein down the board to Kathleen(elder).

Some big differences i think-
Elinor is under no illusions that she is engaged to Edward. Mrs Dashwood and Marianne may expect it, but Edward has never seemed to come close in that respect, though their mutual attraction was evident to those nearby.

In contrast, everyone thinks Marianne and Willoughby are engaged. Some of it is Marianne's fault, but Willoughby is much worse. Contrast the locks of hair. Willoughby begs for a lock of Marianne's hair. Elinor is flattered to think that possibly Edward has stolen a lock of her hair somehow. But he hasn't! This was Elinor's fancy. Yes, Edward, being engaged, was wrong to hang around Norland after he was attracted to Elinor. He was wrong to come to visit them at the cottage too.

Elinor's situation does gain by comparison. Neither she nor Marianne will have the man they love, but Elinor knows Edward would never act the way Willoughby did at the party, and that he wouldn't be capable of writing such a horrible letter.

Elinor's indignation after reading the letter where every line was an insult...proclaimed it's writer to be deep in hardened villainy
...
that she dared not trust herself to speak, lest she might wound Marianne still deeper by treating their disengagement, not as a loss to her of any possible good, but as an escape from the worst and most irremediable of all evils, a connection, for life, with an unprincipled man, as a deliverance the most real, a blessing the most important.