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 nature and excitement about the concert, while contrasting it with Marco's skeptical and cautious perspective. It effectively showcases their differing viewpoints and establishes their personalities in a relatable and engaging way.