Singers or Composers?

I am writing a speech (essentially a research paper) over the great american songbook and I can't decide wheather I should I do it over the singers or composers. I think I can find a bit more about the singers than I can the composers. I think I am going to make my central idea or thesis their contributions to American Music. So what should I do? The singers I have in mind are: Frank Sinatra, Ella Fitzgerald, Nat King Cole, and Billie Holiday. The composers I have in mind are: The Gershwins, Cole Porter, and Duke Ellington. Please give me your input. Thanks.