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 highlights Sadie's optimistic enthusiasm contrasted with Marco's skeptical nature. Sadie's excitement is palpable as she looks forward to the event, while Marco's cautious response illustrates his tendency to worry and doubt, showcasing their differing outlooks on life.