Zigong asked: “Who is better: Zizhang or Zixia?” Confucius said: “Zizhang overshoots the mark and Zixia falls short of the mark.” Zigong said: “Then Zizhang must be better?” Confucius said: “Both miss the mark.”
This is perhaps not the most diplomatic of questions from Zigong, but Confucius handles it fairly.
Zizhang was well known for his quick thinking and great intelligence (and, to some at least, his arrogance) while Zixia was much more dogged in the way he did things. As a result, neither of them hit “the mark”.
After Confucius died, Zizhang and Zixia set up their own schools to pass on his teachings. Their rivalry is described in Book 19 of the Analects.