The difference is 50 is talking about one women, where as Eminem singled out a whole race with his line; "Black girls are bitches."
None the less, if a white rapper said "this black bitch says I have a small dick, but she gotta pussy about this big" he would be flamed left right and center. 50 Cent will not, and the reason for that is, racism against white people by black people is accepted (I hate to use the term reverse racism).