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 choice effectively contrasts Sadie's optimistic excitement with Marco's skeptical nature, illustrating their differing outlooks and enhancing their character development.