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 captures Sadie's optimistic excitement contrasted with Marco's skeptical outlook. It clearly delineates their character traits through their different reactions to the news of the concert, showcasing Sadie's enthusiasm and positivity and Marco's tendency to doubt and expect the worst.