I agree 100% with akannan's assessment. Hughes is definitely better known (including more widely read and taught) in most circles than Richard Wright. The answer to you question, of course, probably comes down to how you define "success." Is "success" the same as "well known"? Or do you have other criteria in mind? Similarly, if you look at poetry alone, Hughes wins hands down, but if you focus on prose, including book-length works and essays, and if you survey people who are very familiar with the full range of African American writing, I think that you'll find that Wright may be seen as slightly more influential among that group of readers.
This is going to be subjective. I think a case can be made for either being successful, or both representing different aspects of the same end on the successful spectrum. I also think it might come down to a matter of personal preference. I think that Hughes is probably more recognized than Wright. This is not reflective of anything else than public appeal. Hughes focused his works primarily on the Black predicament in America of the time. His contemporary, Wright, took the same predicament but also sought to inject a definitive element of class and intellectual understanding of class based conditions into his rendering. I think that Americans, who have always not fully been able to articulate class based conditions and contexts with the greatest amount of ease in vocabulary, found Hughes work easier to understand because it primarily dealt with race and racial conditions. Black Americans could identify with this on an immediate level, and Hughes' skill and mastery allowed many White Americans to understand this, as well. Wright took the same ideas, but injected the struggle of the working man into this dialogue. His travels to Europe and strong alignment with European thinkers who advocated existentialism, class based identity, as well as the different schisms in the proletariat retelling might have caused Wright's message to be obscured a bit. I think that for this reason, Hughes is probably better known than Wright. Yet, in no way should this be meant to say that one is better than another. I think that both writers are supreme thinkers and challenge the conception of what is and allow us to envision what can be.
What an interesting, although clearly a purely academic, question. Are these two authors compared merely because they were both black? That seems a weak basis for comparison.
However, just as a parlor game, I believe that Hughes, because he specialized in shorter works, is more influential because he has been more widely anthologized for younger students. Therefore, most Americans are exposed to him some time from grades 1-12. Some works that appear in almost every schoolchild's life are "Thank You, Ma'am," "I, too, Sing America," "Mother to Son," and "Theme for English B." Wright's work is read in some , by no means all, high school classes. In terms of quality, Hughes was uneven, I believe, compared to Wright, whose stern prose is consistently briliant.