If by Plex you mean downloading BD rips, both consoles suck ass because they do not support DTS and pretty much every quality rip still uses DTS or DTS-MA for audio. Not to mention that Plex transcodes the video as well. They are so bad for this purpose compared to what the PS3 could do that I switched to a Kodi box and never looked back. The Nvidia Shield TV seems vastly overpriced compared to just using a Fire TV, but I haven't tried either one personally. Planning on getting a Fire TV soon though. [Also note that there is a Kodi app for the xbone, but it has the same audio restrictions that Plex and whatever else is out there does.]
Last I checked (which was a long time ago, admittedly) the YouTube app on PS4 was garbage. Not sure about xbone.
Netflix, Amazon Prime, Hulu, etc. are all fine on consoles.