To be precise, Kodi is not a 'server and client' solution.
Kodi is a media player with access to network and the ability to build a media library based on a combination of networked and local file systems. But it's not a server, as it can not share its media library with any other playback clients.
Thus Kodi is suitable for cases where you ONLY want to play media on the same device that holds the media library, as no server is needed for this usage.
A Plex server is motivated when you need to access media from another device than the one holding the media library.
And for these cases it's sometimes possible to also have some Plex client program(s) on the same device, though that depends on the OS used for both server and client. And as stated above, RasPlex uses an embedded OS that currently does not allow for a PMS server.
For those running PMS under some Linux distro for the RPi, it may be possible to use Kodi as a Plex client through one of the available addons for that purpose. Three such addons exist that I'm aware of, those being "PlexBMC", "PlexKodiConnect", and the official "Plex for Kodi" addon from Plex Inc, though I believe that one is still in a preview 'PlexPass-only' release state.
Best regards: dlanor