There both open source. You can play HD-DVD & Blu-Ray with xbmc? You can use external players with xbmc? You can use image mounting with xbmc?
MediaPortal's roots lie in the XBox Media Center (XBMC) project which goal was to build a media center application for the Microsoft XBox gaming console. Erwin Beckers (also known as Frodo) concluded early in the XBMC project that there were notable limitations to the xbox platform, mainly that it is unable to support TV cards. XBox Platform limitations like these would ultimately hinder the vision of a feature rich media center application.
In February 2004, Erwin left the Xbox Media Center project to start a new project, now known as MediaPortal that would run on the Microsoft Windows platform. Initially existing code was re-used from the XBMC project but after several releases and innumerable feature enhancements there has been almost a complete re-design.
Today, MediaPortal continues as an open source project, has a core team of developers and a growing community of over 60,000 registered users.
In 2006 MediaPortal enters a new aera with the release of the TV server. For the first time users can now use multiple frontends for viewing & recording TV streamed from 1 or more TV servers.