I think there is a similarity, but also a huge difference - as you outline.

If we had a graph with two crossing axes - one of openness and reserve, and one of intelligence and insipidity (not sure that those are the right poles for that one, but let's use it as working model), then Elinor and Lady M are at similar positions in relation to the first line, with Marianne at the opposite end. But they are at opposite ends of the second, where Elinor is the same as Marianne. Brandon is in a similar place on both to Elinor, while Willoughby is with Marianne on the first. I need to think a bit more about that second line, because I think it encompasses more than intelligence/insipidity - maybe sincerity, and active engagement with other people? Willoughby's nasty remarkls about Brandon start to put him somewhere at the 'wrong' end of such line, don't they? And Marianne looks in danger of being swept along with him.