Denzel is a GREAT actor and he gets chosen for GREAT roles. He is seen as more versatile because of the quality of his films
But Samuel L Jackson has more range

. Range = getting out of yourself and portraying a character.
Denzel would NOT be a believable Uncle Tom slave like Samuel L in DJango. Denzel was never a badass like Samuel L in Pulp Fiction. Denzel could not pull off that character in Black Snake Moan
Denzel gets much better roles in much better movies, but most of his characters seem very closely tied (lots of similarity between his characters in his "action films" ). Denzel's characters seem like an extension of himself. Samuel L seems like he is playing a different person in each film.