Symphony no. 88
One clear distinction between Haydn and Mozart is that Haydn prefers slow introductions. Also, in expositions written by Haydn, predictable repeats of themes don't occur. Conversely, Mozart is neat and tidy. Haydn emphasizes the "process" of music and how it changes and evolves throughout a piece. Everything is developmental. Mozart, on the other hand, is a composer of symmetry. He emphasizes "balance."
Another great example of Haydn's style (as contrasting with Mozart's) is his Symphony no. 13 in D Major.