This started off as a post about Jane, but since Jane is always talking to Elizabeth, Elizabeth worked her way in at the end.
I've noticed something interesting. Jane is a bit like her mother in the use of imperatives, in stating how the world will be, though hers are directed mostly at herself and when they are not, the motive is very different. These extracts also back up JA's description of Jane as firm where she felt herself to be right (chpt 12):
He is just what
I did not expect (chpt 6)
It is, in short, impossible for us (chpt 17)
You shall hear what she says."
I will read you
But I know the foundation is unjust (chpt 21)
I will not repine
He will be forgot
My dear Lizzy, do not give way
You do not make
I must think
But enough of this
I cannot believe it.
Do not distress me (chpt 24)
Jane's speech comes across as on the whole calm, and firm - as on the whole the phrases up above indicate.
"Laugh as much as you chuse, but you will not laugh me out of my opinion. (chpt 17).
Looking back, the sentences in chapter 4 indicates Jane's excitement over Bingley by the (so far) rare use of exclamation marks in Jane's speech:
I never saw such happy manners! -- so much ease, with such perfect good-breeding!"
"Dear Lizzy!" (chpt 4)
And again, the thought of Bingley raises an exclamation mark
Can his [Darcy's] most intimate friends be so excessively deceived in him? -- oh! no." (chpt 17)
A thousand things may arise in six months!" (chpt 21).
I wish I could localise what makes Elizabeth's speech so witty in comparison to Jane's (on the level other than JA's choice of course). I think there is something in that Elizabeth pairs surprising ideas together:
"You must decide for yourself," said Elizabeth; "and if, upon mature deliberation, you find that the misery of disobliging his two sisters is more than equivalent to the happiness of being his wife, I advise you by all means to refuse him."
"How can you talk so?"(chpt 21)
Elizabeth also states bluntly what is going on socially:
Miss Bingley sees that her brother is in love with you, and wants him to marry Miss Darcy. She follows him to town in the hope of keeping him there, and tries to persuade you that he does not care about you." (chpt 21)
Elizabeth also uses an irony that, as far as I can tell, Jane lacks. Jane is always serious by contrast in her speech (all quotes from Elizabeth:
Believe her to be deceived, by all means. You have now done your duty by her, and must fret no longer." (chpt 21)
Stop me whilst you can." (chpt 24).