The dialogue that would best help develop both characters for readers is:
“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 emphasizes Sadie's optimistic outlook and excitement, contrasting sharply with Marco's skeptical and negative attitude. It clearly showcases their personalities, with Sadie dreaming big and Marco's doubt disrupting her enthusiasm, making it an effective development of both characters.