I prefer infuse to plex in every single way except for one.
There's one thing Plex can do that to my knowledge infuse can't and it's something I find terribly useful.
I use a projector so when I just want to listen to something like my surround sound mixes of Dark Side of the Moon or Wish You Were Here for example I tell Siri to open Plex on my ATV4K. Then I open Plex on my iPhone and I can navigate the TVOS Plex using my iPhone and what I play back is not "cast" to the ATV4K but it's played directly using the ATV4K.
Maybe this can be done and I just don't know how to do it. If I could do this with infuse (I bought 6 Pro standalone for now) I would have no need for Plex (Aside from the fact it's actually tremendously useful as an organizer of a library for infuse.)