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Because I like to have a graphical interface? Like this:

Not sure what the difference is really. PS3 provides a 15 second clip of the movie file alongside it in the menus, and you can change the clip if you want.
 
I have an ATV and a PS3, and there is no comparison between the two interfaces. Sure, the PS3 works for playing movies and music, but it is clunky and just plain ugly. The ATV is just a lot nicer to use. The only thing I like about the PS3 is the ability to just plug in a drive and watch movies. I wish Apple would just enable the ATV's USB port.
 
Ended up getting the previous generation high end 2.0ghz Mac Mini new in box for $440 on eBay minus Cashback.
 
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