Hi! Things here are rather rushed, so this bit is off the top of my head (and from memory) which may give you a start. Darcy's first proposal was in the form of a letter and therefore heavily considered. It insulted Elizabeth's family, but he offered to overlook that at huge personal cost to him. Had she accepted then she would forever have been subservient to him and grateful to him - she would probably have gradually cast off her family. It was very serious, painful and dramatic.
Mr. Collins, on the other hand, has for many years been to me the fount of comic embarrassment. He had not considered that his social position was lower (NB - Darcy considered Elizabeth as lower) than Emma's. He blurted it out, seizing his chance in the carriage and could not offer any social advantages at all in reality. (Here's where I need to go, re-read and check, sorry.)
So, comparison is between socially unequal proposals from two ends and then there is the difference between considered language (written) and spoken - both are unacceptable. Hope this give you a guide.