Another huge benefit of Plex over the other streaming options is having a central media library serving the "thin" clients. With XBMC, I have to configure each installation as a stand alone setup. It has to parse, scrape, and download all of my movie and tv show information, download artwork etc. It's a lot of work to do if you have a large collection and want to setup several apple TVs for example.
With Plex, you set up your library once, in one location (the server) and it's trivial to point the clients to it. If changes or additions need to be made, you make them in one place, the server. This aspect alone makes Plex leaps and bounds more useable for me compared to the other options.