I prefer Kbin. It has the latest features (magazine collections, an aggregate view that combines threads and microblog posts, more options, etc.) , and I really like the developer @ernest. Aside from just being cool, he posts frequent updates over on @kbinDevlog.
Something worth noting is that Kbin doesn't have many instances. Kbin.social is the main one, and the only other decently up-to-date one I know of is kbin.earth, run by the creator of the Interstellar app for Kbin. On the other hand, Mbin has more instances, as most established Kbin instances switched to it at the time it was created (during which Kbin development had been inactive for about a month). Also, Mbin has a more community-oriented development system tmk, whereas Kbin development is closely managed by Ernest.