Plex has two components, a server that runs on a computer which has all your media on it and the client. In this setup the client is on Roku, but you can also get it for Android and fire tablets, iOS.
The biggest hurdle to Plex is getting your video downloaded. That’s a moral and possibly legal sorespot for some.
Plex handles the show and movie art and synopsis, groups by season etc automatically. You sit down with your Roku remote and launch Plex and browse automatically sorted and categorized beautiful looking lists of your files. Pressing play causes video to play in 2 seconds or so.
Kodi is harder on the setup side. It isn’t really hard but it is so different than the way you naturally load apps now it will take maybe an hour the first time you set it up or update it.
Media is as easy to find as a quick search BUT you need to have good kodi plugins installed. These plugins loosely mirror websites that aggregate content hosted on servers around the world. The list of servers each plugins uses isn’t updated or maintained particularly well. Also everything on kodi streams. So you’re at the mercy of your ISP and the hosting site as well.
Kodi plugins have episode guides and cover art and handle that automatically as well. My personal preference with Kodi is a keyboard and track pad, it could be tough with a remote only.
I can’t actually figure out how kodi even exists. There are no ads, these sites that host the content stream tens of thousand of times a day … This results in broken links and bad streams that stutter and such. After a few tries you’ll usually find a working link. But for things less popular you could be out of luck.
Kodi is also tough to find hd streams of anything but the most popular stuff. Plex is only limited by your source material, so if you rip your own hd content or download it you can play at native hd up to whatever your wifi network will facilitate.