I would say that one of the fundamental differences between the two expressions of African- American art has to do with historical context. For the thinkers of the Harlem Renaissance, the articulation of what it meant to be "black in America" was a newly spoken idea. Its words, lexical patterns of recognition, and ideas were in the infancy stage of emergence. Black writers during the Harlem Renaissance were amongst some of the first thinkers to put into words what it means to be "the darker brother." This articulation was filled with a wide range of emotions, but it marked one of the first times where a movement of black thinkers and artists were discussing the same topic and the divergent points of view within it. The time period was one where the experiences of the First World War and the location of fame and notoriety in America was only beginning to develop. Accordingly, like all first steps, small ones were taken. Significant, but the developing of walking was critical.
The Black Arts Era of the 1960s and early 1970s was a product of "Black Power" and the need to assert with force the identity of Black Americans. The anger and social activism that was so intrinsic to the time period came out in the Black Arts Era. The developing steps of the Harlem Renaissance had resulted into a full out spring, with long and defiant strides taken in order to bring the idea of change and transformation into full form. The idea of "intense revolutionary fervor" was apparent in the Black Arts Era because the time period was advocating such demands on all different levels.
I think that philosophical scope of reference in each articulation was dictated by the time period. The Harlem Renaissance began the process of calling out for the need to change, while the Black Arts Era demanded it. The Harlem Renaissance spoke about what it means to be silenced, while the Black Arts Era demanded that voices will be raised and heard. The Harlem Renaissance came to grips with the idea of being marginalized and this sadness settled into its works, while the Black Arts Era moved sadness into anger and demands for change.