I just watched S&S3 (for the first time!) last week and I was surprised when Marianne said on the way home from Cleveland,

"It's not what we say or feel that makes us what we are, but what we do, or fail to do."

I don't think that line is anywhere in the text but I thought it rather fitting to the story. She was looking out the window (at the Col) of the carriage, so she may have been comparing the Colonel to Willoughby, or herself to her sister.

Marianne during her illness was able to reflect. She had prided herself on her strong feelings, yet she was behaved rather coldly to others whom she didn't like. Contrast that with Elinor who was always solicitous of others' feelings in her behavior.

The Colonel was active in relieving the suffering of others, Eliza I and II, Edward, Marianne and Elinor, compared with Willoughby who outwardly was had this open and affectionate manner, who turned out to be very unfeeling in his behavior.

It also reminded me of the comparison we already had in the GR of the good-hearted Sir John Middleton with John Dashwood, who made a promise and professed to his wife his desire to help, but was very comfortable with not doing anything at all.

Are there any other comparisons to be made in the story?
Mrs Jennings and Mrs Ferrars?

I rather liked the quote, though I would say that it is important what we say or feel also. One would hope that people do right for the right reasons, and not just because it is what they should do. But in the story the characters who "do right" seem to have good hearts also. Now the more I think about it though, our good-hearted Sir John and Mrs Jennings do not always "say right" either.