I believe it is more humiliating for the black man. This is because many people expect the black man to have a BBC. But if he is sporting only a 6 and maybe even up to a 7 inch cock, it can lead to a significant sense of disappointment. After all, a lot of white guys have 6 and 7 inch dicks.
By contrast, many people often assume that white men will have a smaller cock and so there’s generally less potential for humiliation when those expectations are true.