It makes sense that Ragna would find Mirajane and her ability to read people scarier than Erza. Even leaving out that he's probably strong enough not to fear a fight with either of the S-Class mages, in his past experience he's mainly been screwed over by the settings Chessmastersoutmaneuvering him to render almost anything he does meaningless by the time he confronts them rather than simply being overpowered by stronger opponents. Someone who has a talent for manipulating others (even a complete Nice Girl like Mirajane) would register as a bigger threat to him than a powerful, but straightforward fighter like Erza.
