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 effectively showcases Sadie's optimistic nature as she jumps at the chance to celebrate the band's performance, while Marco's skeptical personality is highlighted through his cautious response about the likelihood of getting tickets. Their contrasting viewpoints help to clarify their character traits and establish the dynamic of their friendship.