The next step up would have to be XBMC, which is simple to set up and a breeze to use. It also requires minimal system resources and runs exceptionally well on low-end hardware, especially the Raspberry Pi.
MediaPortal and Plex would be well suited for more savvy users who don't mind going through some additional installation and configuration steps to customize their experience. MediaPortal and Plex and are both powerful front ends that offer a number of advanced features, over and above basic media playback capabilities. When you're ready to take your home theater PC to the next level, MediaPortal and Plex are the best tools to help you pull it off.
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