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 response effectively showcases Sadie's optimism and excitement about the upcoming concert, while Marco's skepticism and tendency to expect the worst are clearly illustrated in his contrasting response. This exchange highlights their differing perspectives, enhancing the reader's understanding of both characters.