Vincent van Gogh's work was Post-Impressionistic. Claude Monet's work was Impressionist. Monet's work was very "soft-focus", while van Gogh's work was a unique style no one has replicated, and more like he was "drawing with paint" as my figure drawing professor once said. Monet's work was about showing the light, while van Gogh's was about the subjects of his work. That's my own impressions about them, not anything from an art history text.
What I see when I study the two of them, both of which vie for my favorite artist, is that Monet used a shorter, squarer, stroke in his work. Van Gogh used long streaks of paint to create his work.