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!”
“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 both Sadie's optimistic nature, as she excitedly anticipates the fun they will have, and Marco's skeptical and brooding character, as he immediately points out the potential negatives. This contrast in their perspectives helps readers understand the dynamic of their friendship and the personalities of each character effectively.