I am not entirely certain that there is a "versus" in between both concepts in Of Mice and Men. Part of Steinbeck's genius is to present a world of what can or should be, a world where loyalty and friendship are the basis of human interactions. Lennie and George demonstrate how human beings should behave. This creates a sense of awe in the other characters. One of Slim's first remarks to both men is how no one "travels together" any more because people are "afraid" of one another. Candy is enticed by the vision of loyalty and friendship that Lennie and George display, so much so that he wants to be a part of it. Crooks sees Lennie and George with their display of loyalty and friendship and it reminds him of the pain in his own life as he lives without both. In this regard, Steinbeck is able to depict an emotional world where loyalty and friendship can converge. They go hand in hand, as people walk on in a world filled with despair and loneliness.
George and Lennie carry themselves in a way where friendship and loyalty work with, not against, one another. The friendship both men have causes them to be loyal to one another. Lennie adheres to George's demands. Even when the thought of trouble to George is conceived, Lennie reacts violently. Crooks experiences this in a first hand manner. When George tells Lennie to do something, such as keep quiet or fight Curley, Lennie is loyal to his friend. In the same way, George is loyal to his friendship to Lennie. George sacrifices everything in his own life for Lennie, his best friend. George remarks on this with his "When I think of the times I could have had" moments. He also demonstrates this in the final moment of the book, when he has to do what he must do because of his loyalty to Lennie and his friendship with him. Loyalty and friendship are not mutually exclusive. They are not a choice that Steinbeck is forcing on his characters or on us. Rather, they are an active construction which shows that if individuals believe in another, they will act with loyalty and friendship in mind. In the midst of the desert that is the Great Depression, Steinbeck offers a garden of hope where loyalty and friendship are the seeds through which flowers bloom.