I am a huge fan of the Plex/Roku combination. I’m exclusively Ethernet, but I can see the usefulness of having a Roku stick in my bag for traveling (and will probably pick one up for that purpose.)
Plex/Roku has a big WAF which is also a plus.
If I want to show off a YouTube video, I PlexIt! To my queue. Have RSS feeds thrown to my queue via IFTTT. It can access my EyeTV queue and my HD HomeRun tuner.
To be honest, I’m not sure–but I suspect that it’s both (or it depends, take your pick). I have each Plex client to play direct when possible, but it downgrades (transcodes) on slower connections or devices with less horsepower.
Plex agents aren’t half bad at scraping metadata, but they’re by no means perfect.