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.”
“We’re going to have so much fun,” Sadie said. “I bet we’ll get to meet the band. I bet we’ll even get to hang out with them!”
“We probably won’t even get to go,” Marco said. “I bet the whole show will get canceled and we’ll have a terrible day.”
This exchange effectively demonstrates Sadie's optimistic and enthusiastic nature contrasted against Marco's skeptical and pessimistic outlook. Sadie's excitement and positivity shine through in her dialogue, while Marco's cautiousness and tendency to expect the worst are clearly articulated, showcasing their differing personalities.