The dialogue that best helps 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 clearly showcases Sadie's optimism as she enthusiastically anticipates the fun they will have, while Marco’s skeptical nature is highlighted by his negative outlook on the possibility of the event occurring. This contrast effectively illustrates their differing personalities.