Plex transcodes instead of just serving up data, which is useful if you have older devices that aren't able to read the newest codecs. That said, you need to make sure you have the bandwidth to support near-raw video streams, because it's really really bandwidth intensive.
I ran Plex for a second back in 2009 when it was first forked off XBMC and didn't like it; it was slow, a resource hog, and my Mac would get boiling hot trying to decode and stream all the data, and it really didn't understand the codecs well (basically decoded more than it should have).
I just run XBMC (now Kodi) off a HTPC into my receiver, streaming from a NAS and it does really well. My receiver has multiple HDMI outs, so I can play content on any of my two TVs.