The dialogue that would best help develop both characters for readers is:
“No way!” Sadie beamed. “Clear your schedule. We’re going to have a blast.”
“I don’t know,” Marco said slowly. “Are you sure we’ll even get tickets? This is going to be a pretty big deal.”
This exchange showcases Sadie's optimistic and enthusiastic personality, emphasizing her excitement for the concert and her tendency to look on the bright side. In contrast, Marco's response reflects his skeptical nature, expressing doubt about the feasibility of getting tickets. This contrast effectively highlights their differing outlooks and deepens the reader's understanding of their respective characters.