For anyone wondering, which mentor you get seems to depend entirely on your dialogue options with Quirinus, except the dialogue choice that gives you a stat gain. My first playthrough I got Cenric and decided to start over, since Cenric didn't fit with the stats I want to focus on. My next playthrough, I kept everything the same except for what I said (or didn't say) to Quirinus, and that time I got Varyn, who I was hoping to get.