This is not as easy as many people make it out to be because looking at it from the point of view that both Shanks and Mihawk are swordsmen, there is no debate. There is nothing shanks can with a sword that will make him stronger than Mihawk.
The Problem is that since we have been introduced to Shanks, he hasn't done anything significant with a sword (aside from blocking Akainu). What has been used to hype him has been Conqueror's haki.
It has been used on three separate occasions: Chapter 1, on Whitebeard's ship, and scaring off Aramaki in Wano. When one thinks of Shanks, his sword is not the first thing that comes to mind, it's his Conqueror's haki.
The point I'm trying to argue is that Shanks is a has-been swordsman that lost an arm years ago, to compare him to Mihawk on the basis of swordsman is simply unfair, especially since the only hype he has gotten since losing his arm has been because of his Conqueror's haki.
Oda even hinted that may be more to how Shanks fights in an SBS, until he is flexed out, we can't judge how strong he is.