It goes either way at this moment, Seju or Sungji, both are possible. Now we have to wait to see Sumin's feelings in the future chaps to know who will be the ending couple. However, I lean more on SuminxSungji endgame (for the sake of my bet, lollll) because well, it seems to be that way in many reasaons, lol.
But well, the conversation is beautiful, btw. They both live and grew up in opposite ways, Sungji with love, care and friends, she never experiences hatred or loneliness so to her, she couldnt understand Seju before, until Dawoon came and said those words, Sungji gradually realized how it feels to be in Seju's shoes, just the gist of it, not all. This chap, we finally understand Dawoon's role in the story, and Foxy Lady is right, Sungji remembers Dawoon's words. Seju, on the other hand, never feels love, it's all about hatred, so the world Sungji is in, she never really understands. They both realized how much different they are this chap, in their thoughts and words,
they put themselves in each other's shoes(or maybe only Seju, Sungji has realized a few chaps ago, maybe?)
To be really honest, this conversation makes me want them a couple, not SejuxSumin, not SuminxSungji, its SejuxSungji. Well, can't believe till the end, I could honestly ship a couple in wdtfs and that's a dead-end ship, lol.
btw, this chap, it's not about who is mature or not xDDDDD, it's about understanding.