First, thought: many of Li Bai's poems show the idea of pursuing immortality and eating, drinking and enjoying in time; Many of Du Fu's poems show the thoughts of worrying about the world and caring for others. So Li Bai is called "Poet Fairy" and Du Fu is called "Poet Saint".
Second, style: Li Bai often uses exaggerated techniques and vivid metaphors to express his warm feelings; Du Fu often shows his deep sympathy for social reality with vigorous skills and profound thoughts. Li Bai tends to be romantic, while Du Fu tends to be realistic.
Third, the genre they are good at: both of them can write poems of various genres, but Li Bai's seven-character ancient poems and quatrains are the best, and Du Fu's metrical poems are the best.