I currently use both, LMS for music and PLEX for video. Not sure what LMS features you are currently using, but make sure you consider them before ditching LMS. I have about 8 LMS players (classic Slim Devices, and Raspberry Pi's with HiFiBerry DAC's etc. ). I have 3 Nvidia Shields as PLEX players. Both LMS and PLEX are running on the same host as VM's.
I really like that I can synchronize all my squeezebox's throughout the house during parties , While BBQing on the patio , or house cleaning day etc. I also have Spotify configured on the LMS which can stream to any or all of the players. I also like the ability to create playlists which can source music files both from Spotify and my local library. Now I am not sure of the scaling limit's PLEX has , I currently have over 1100 movies and it does not seem to be having any issues, but the LMS library is currently sitting at 90k FLAC files and running strong. I also like the fact I can spin up a new "squeezebox" with a really decent DAC for a $150 or so. One other thing is the material skin plugin for LMS and using my phone as my remote control for all 8 "Squeezebox" players. Now PLEX may have answers for all these features that I am not aware of, but might want to verify that if you use any special LMS features.
Cheers,