Btw, Plex is heavily based on XBMC. The current version of Plex Media Center is based on XBMC 10.0, which is now very dated and has some playback issues. Plex Home Theater will be based on the just released XBMC 12.0.
The major difference between the two is that Plex uses a central server application called Plex Media Server designed to easily distribute media to your HTPCs, phones, tablets, DLNA devices, Rokus, and more. The other major difference is that Plex and XBMC use completely different plugins, and they aren't cross compatible. I personally prefer the plugins for Plex, just because whenever you add a plug-in to one device, it's automatically added to every device. XBMC probably has more plug-ins to chose from overall though. Overall, media management is far superior or Plex and easier to setup, and it allows you to access your media from almost any device, in and outside your network (so, you could stream your blu-ray from your server to your phone, even if you're halfway across the world) . Lastly, Plex does not have live TV and DVR support like XBMC 12.0. ISOs aren't going to work well with Plex either.