Swordsmanship. It's not that complicated. Mihawk is the WSS. Prior to this chapter, it was not known of Shanks or him won their duels, but this chapter at least makes it clear he is the better Swordsman.
So the question really boils down to: is Shanks' prowess only related to Swordsmanship? We can see he made an Admiral run purely from Haki alone, but he's only actually fought with a sword
I guess that question will be up to Oda.
For the record, I have no issues with Mihawk being stronger or a better swordsman lol, its really irrelevant to my conversations about Shanks. What I care more about is his goals and potential antagonism. If he is as complex as Oda insinuates he is, and has an established conflict with Luffy, I could care less if he's the next person to go down or the last. I just want a good character lol.
Zoro vs Mihawk is 100% Zoro's last fight. The REAL question is....who's Luffy's?