To the shock and amazement of both the men Elizabeth rejects their proposals summarily. Collins of course thinks that she is acting coy till she tells him that it would be "absolutely impossibe" to marry him. It is only after this firm rebuff that he becomes romantic and praises her :"you are uniformly charming!"
Darcy on the contrary becomes "pale with anger" and wishes to know why she was rejecting his proposal. Eliabeth immediately gives him three reasons for rejecting his proposal: prevented Bingley from marrying Jane; spoilt Wickham's chances of advancing in life and his arrogant and selfish nature. She concludes by saying that he would be "the last man in the world she would marry."
Elizabeth, unlike Charlotte, does not compromise her principles and remains true to her feelings till the end by refusing to succumb to financial pressure in matters concerning her marriage. She tells Collins that she is not an "elegant female" but a "rational creature speaking the truth from her heart."
Collins proposes to Elizabeth on Wednesday November 27th at her own house (Ch.19). Darcy proposes to Elizabeth at the parsonage at Hunsford during the Easter season when she was visiting her friend Charlotte who is now married to Mr.Collins (Ch.34).
Collins is a cousin of Mr.Bennet who will inherit Mr.Bennet's estate after his death. This is why he is so arrogant and confident that Elizabeth will not reject his proposal. Darcy is also a very proud man because of his immense wealth and his large estate and he is also confident that Elizabeth will not reject his proposal. Both of them take Elizabeth for granted and impress upon her that they are actually doing her a great favour by marrying her and try to exploit her financial distress to their advantage. They do not care to find out leave alone respect her feelings with regard to marrying them.
Both of them are completely unromantic men. Their arrogance prevents them from praising her beauty or her intelligence or flattering her before seeking her consent.
In Collins' case Elizabeth was aware that Collins had decided to marry her as early as Ch.17. but she never had any inkling that Darcy was interested in her and his proposal takes her completely by surprise.
Collins gives three general reasons why he wants to marry without specifying why he wants to marry Elizabeth in particular. Darcy blurts out his love and admiration for Elizabeth in particular.