Plex has 2 main advantages over Kodi for most people.
1 It has a lovely server backend that keeps track of where your at in shows and what you have watched , arguably its scrapers and meta data work better but that is indeed arguable.
2 It plays anything you throw at it.
However point 2 is a big part of the issue in that anything that Plex does not like it will transcode sometime subtly altering quality and sometimes not so subtly.
Note simply setting Plex to Direct play will only stop this IF plex recognises the format and plex can be pretty poor with not liking formats.
Frankly for most people this does not matter one bit , a small reduction in quality is just that a small reduction.
Set up perfectly with all your media in the same format there is no reason why Plex can not produce the same quality as Kodi but when stuff is not correct it is much easier to sort with Kodi.
Then we get to the Kodi configuration options for expert users the sort of things that smooth out audio timing issues and 24p syncing , honestly there are whole forums devoted to this stuff and it is much more configurable in Kodi than Plex.
Back to that original quote !
If you cannot tell the difference then do not worry about it, sometimes the enjoyment is not seeing the imperfections.
