You've made me think. We have been told exactly what the characters' motivations and inner lives are like from the beginning.

With S&S we get a lot of our understanding of the characters filtered through Elinor's thoughts.

In MP, we're told by the narrator what Henry and Mary are up to the moment they arrive. We are told that Maria, Julia, and Tom are selfish, lest we should be confused by any outer propriety of manner.

There is mystery in S&S with Willoughby and Edward, but in MP all is laid bare. We get to see how their actions fit in with what we've been told about their inner character. Can these people change?