こと vs の (nominalizers)

The two main nominalizers こと and の, both turning a clause into a noun-like unit. の is concrete and colloquial, preferred for direct perception and experience (見る, 聞く, 感じる take の: 彼が走るのを見た 'saw him running'); こと is abstract and formal, preferred for propositions and general statements (思う, 信じる, 知る lean toward こと, and ~ことがある / ~ことにする are frozen with こと). Often interchangeable, but each verb class has a preference.