Musial was a more complete player than Williams. He was an excellent fielder and base runner too. Williams was not a good base runner and his fielding was just adequate.
Edit: The debate has always been DiMaggio vs. Williams, because they played in the bigger media market. Musial belongs in the debate. He was a better all around player than Williams, and he was versatile playing 1st and OF at an excellent level. He was a better hitter than DiMaggio. Williams was the best hitter of the 3, but he was one dimensional compared to the other two. They were the 3 giants of the generation, but you hear very little about Musial.