The dialogue that would best help develop both characters for readers is the fourth option:
“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 highlights Sadie's optimistic nature as she expresses excitement and positive expectations about the concert. In contrast, Marco's response emphasizes his skeptical and brooding character, showcasing his tendency to predict negative outcomes. This contrast effectively illustrates their differing perspectives and adds depth to their personalities.