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 choice effectively showcases Sadie's optimistic personality as she excitedly anticipates the fun they might have, while simultaneously highlighting Marco's skeptical nature as he downplays the chances of a positive experience. The contrast between their views is clear and encapsulates their character traits well.