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 effectively highlights Sadie's optimistic and enthusiastic nature, contrasting it with Marco's skeptical and pessimistic attitude. It clearly establishes their personalities and their dynamic as friends, enhancing the readers' understanding of both characters.