It's always been ironic to me that Elizabeth, the brightest and most savvy of the Bennet girls, so clearly sees Mr. Collins's proposal as loveless and merely expedient--yet she fails to recognize that Mr. Darcy's offer of marriage is genuine and based on love.
Both proposals are rather abrupt and even business-like; both offers are rebuffed by Elizabeth, though she is perfectly content with that answer only in the case of Mr. Collins. After her ranting and raving (and Darcy's thunderous leave-taking), Elizabeth is forced to reconsider her position and face her own emotions when she receives a letter from her future husband. She has no such second thoughts and undergoes no emotional examination after the first proposal--because, of course, there were no emotions involved.
Both offers are unexpected, in part for the same reason--before their proposals, Elizabeth had no indication that either man feels any kind of love for her. Her exchanges with Mr. Collins were insipid compared to the fireworks which inevitably resulted from time spent with Mr. Darcy, but neither kind of encounter reflected love. At some point, of course, Darcy has realized this is the kind of relationship he wants for the rest of his life--money and position or not. It takes Elizabeth longer to get there, probably because the figurative "step up" for her is more difficult to contemplate than the figurative "step down" is for Darcy.
Elizabeth, who has thought of herself as being relatively open-minded and not as particularly class-conscious, is offered two potential lives. With Mr. Collins, aside from the lack of love, she knows she would always be made to feel as if she should be grateful for the gift of his name. She knew that was not the life for her--and sees it first-hand later in the novel.
With Mr. Darcy, despite his wealth and position in society, Elizabeth eventually realizes she will be his equal in all the ways that matter. It takes them both a while to get there; however, once they overcome their pride and prjudice, theirs will be a match of love.