The dialogue that would best help develop both characters for readers is:
“I hope my uncle can get us tickets,” Sadie said. “I know I’m an optimist, but it could be really hard because the band is so popular.”
“Well, I’m usually skeptical and I still think we have a good chance,” Marco said.
This exchange effectively highlights Sadie's optimistic nature and Marco's skeptical personality, allowing readers to understand their contrasting outlooks and adding depth to their characters.