They both have their fans, so I think it depends on what you're looking for. If you want plug 'n play, it would probably be Plex. If you like tinkering, tweaking and side-loading -- and the resulting added customization options, you'd probably prefer Kodi.
Kodi may have more powerful search options than Plex, but that's not saying much. There's no search in Plex television libraries by actor, only for title, genre, year and content rating. For Plex movie libraries, there's more -- it's the same plus: decade, director, starring actor (only), country, MPAA rating, and resolution.