I would not rate Kaku as a swordsman, but:
Would you rate Zoro at "similar" or "Slightly greater" skill with a sword than Kaku? In which case, Zoro ONLY has swords, whereas Kaku had at least 2 other tools (or skills, however you see it) at his disposal. Despite having a whole host of tools at his disposal, he lost to Zoro.
Thank you for reminding me of this battle, because it is a perfect example for what I've been arguing (and possibly Blackhair and others?). Kaku had plenty of other things to work with over Zoro who only had his three swords. Both had insane physical ability, and combat wise they ended up quite close to each other. However this combat ability was a culimination of their total skills in various tools. Zoro was exceptionally skilled at the sword, Kaku skilled with his Rokushiki (and growing with a devil fruit) and swords. Despite Kaku having extra tools, Zoro was able to emerge victorious. So having extra tools, does not guarentee you victory, or even always guarentee you an edge.
Shanks, like Kaku, has an extra tool. This does not guarentee him dominance over Mihawk and his one tool. Of course, it adds to his combat ability. But unless you can somehow prove to me that Shanks has swordsmanship on any scale comparable to Mihawk (which we can't given we've only see Shanks swing it once), it's impossible to rate Shanks over Mihawk based on haki + swordsmanship.
Of course, I'm not saying Shanks might not have swordsmanship comparable to Mihawk. And then, if he does have haki and Mihawk doesn't on top of that, then he would be greater than Mihawk most likely, or it would still be close.
These are kind of the points I've been trying to get across. Since it's impossible to see a difference on reputation and what has currently been shown thus far, it comes mainly from prediction, and I honestly could not predict to you a winner. I just find too many Shanks arguments ridiculous, and it's obvious many were unimpressed with Mihawk in this war.
In my opinion, they are both end-gamers. Both the goals of the strongest two Strawhat's who are always of comparable strength. I would like Mihawk to be stronger than Shanks on story progression, but it could easily be the reverse.