PS3 is much better than Xbox for media sharing.
Having both systems I have to say I just don't agree. The PS3 has hi-def sure, but my trusty ol' XBox with XBMC is a joy to use (for those that don't realise it, the XBox can do 720p and 1080i by the way). I've found many a video file the PS3 just will not play due to being an unsupported format and the XBox plays them all. For ten quid plus the cost of the drive it stands out as one of the best tech bargains around.
I also watch movies and play mp3 on my old GBA too. Wonderful what can be done with a bit of hardware abuse.
Meanwhile, back on topic. I would love to see a specific XBox AROS build, someone should mention it to buzz.