Idk what Oda is doing with Mihawk and Saint Shanks. If we take Mihawk's title at face value, Mihawk>Saint Shanks.
however
Saint Shanks has the best CoC feats in one piece bar none. Mihawk just doesn't. Mihawk is WSSM, he has better skill with a sword than Saint Shanks, yet Oda can't just come out and say Mihawk defeated Saint Shanks. He can't give Mihawk a higher bounty than Saint Shanks. He can't even have Mihawk split the sky with Whitebeard, despite them both being in marineford.
Mihawk is said to be Saint Shanks's "rival", but Saint Shanks has a much greater rivalry with blackbeard. In fact by his own admission, Teach is more dangerous.
Maybe it's like this. Saint Shanks< Mihawk, however Saint Shanks's CoC mastery> Mihawk's, with Mihawk solely being stronger due to having a black blade and 2 arms.
in any case, I don't think you can say Saint Shanks fans don't have a point, when Oda himself wanks Saint Shanks more than almost any other character.