1st of all, this is really an interesting pick.
2nd of all, DON'T GET ME WRONG. They're both the queens in their respective genre. The reason I picked Aretha was because... well, there really isn't much of a competition in her genre (especially considering Aretha's wondrous talent, nothing managed to top her off). As for Madonna's genre, it's "Pop". Don't get me wrong, she is indeed deserving of THAT title considering her success & her loads of achievements but we're talking about Pop music here, Countless of artists have been successful in that area even before she began (like Cher, although her honorary title is the Goddess of Pop). There have been A LOT of people who have been dubbed as the queen of pop (e.g. Lady GaGa, Britney Spears, Christina Aguilera, etc.) & again, I'm not saying that those artists are in Madge's level in terms of achievements, it's just that with all the artists gaining success & widespread acclaim, it's hard for some people to even say who is the true queen of pop.
My long ass essay may be quite unclear but I don't know how to make it any clearer. :|

^Thanks again for your comment. It is very clear what you what to say and I agree!