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VLC seems to be have a lighter load when it comes to system resources, plus it is a lot faster when playing HD video files. At least that is my experience anyway.

I agree. Also sometimes QT will just not like a file and decide not to play the audio or video part, even though I have Perian installed; in those cases I just use VLC.
 
Not sure, but they should I think. Perian handles almost anything though Qt is quite finicky about mpeg files.
 
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