Zoro asked Mihawk to teach him the way of the sword. Assuming Mihawk can't teach Zoro swordsmanship makes no sense.
What I am saying is that Mihawk cannot use Santoryu. Same way Rayleigh cannot use the Gum Gum fruit. Both of them can provide advice with how to fight due to their vast experience as fighters, but Mihawk's style is nothing like Zoro's style. So I don't feel that Mihawk has any advantage over Rayleigh as a teacher due to being a swordsman.
Regardless why would Rayleigh be a better teacher than Hawkeyes?
-I would imagine he gave the apprentices on Roger's crew advice and training in his role as First Mate, given that we've seen him act at least as a disciplinary figure to Shanks to Buggy. I find it unlikely, given Mihawk's personality, that he's ever mentored anyone before Zoro. Teaching would be new to him
-Rayleigh actually had the plan set up for how to train Luffy, where he would go, how long it would take. Zoro just sort of sprung it on Mihawk as a surprise.
-just their personalities in general. Rayleigh's got the wise old man vibe down to a tee.