If one were to build a case that Malcolm X was less significant than Dr. King, I think that one aspect would be based on national significance. While both leaders were known around the nation, Dr. King willingly had linked himself to the movement. He was willing to embody the aspirations of the Civil Rights Movement, and became the voice that both people of color and White individuals embraced. When Dr. King delivers his "I Have a Dream Speech" at the March on Washington, his voice becomes synonymous with the Civil Rights Movement. It is a voice that spoke to both White Americans and African- Americans. This is something that Malcolm X was never able to do. Partially by design in the early stages of his participation in the movement, Malcolm X did not seek to be the universal voice of the movement that Dr. King was.
Another reason why Malcolm X might be seen as being less significant to Dr. King could also be within the content of both messages. Malcolm X spoke from a position that sought to empower African- Americans. His message was one that spoke to people of color who experienced prejudice, discrimination, and marginalization. This is contrasted with Dr. King who tailor- made a message that spoke to both White people and African- Americans. Pulling from the moral righteousness of Gandhi, King's message was perceived as a universal one, a construction where discrimination represented a moral evil and violence was a stain against one's sense of character and dignity. The humanist point of view that King invoked enabled his message to be accepted by more people, contributing to why his presence was more valued than Malcolm X.
Finally, I would suggest that Dr. King's actions in terms of working through protests and boycotts, being arrested via Civil Disobedience, and representing a realm where theory and practice merged made him more essential than Malcolm X. While Malcolm X was a voice and a very important one in the dialogue, Dr. King's actions were ones that represented "the fierce urgency of now." The ability to take action in the name of those who were less fortunate was something that Dr. King understood and embraced, making him more significant to the movement.
I think that it should be noted that towards the middle and end of his life, Malcolm X understood this. Malcolm X understood that the message of Civil Rights and equality was so important that it did not matter who was the messenger. Malcolm X might have had pointed words for leaders like Dr. King, but towards the end of his life, Malcolm X understood that Dr. King's efforts being geared at a larger entity were important. Malcolm X positioned himself to the White population almost as if to say that they should heed Dr. King's words for if they didn't, people with more pointed and stronger messages such as himself might ascend to power and influence. Essentially, if White America knew what was best for it, there would be a readily acceptance of Dr. King's messages. The seminal picture of both men meeting and shaking hands, eager smiles on both of their faces, is probably the lasting image I take with me, expressing how they are both important even if Dr. King is seen as a historically more significant figure in the Civil Rights Movement.