IMHO the 360 as a media player is one giant joke
It can't handle half the formats or containers the PS3 can and it has a horrible time playing hard disk media. Before I was streaming from my media server I would ALWAYS without fail 45min-1hr into the presentation get a "This media cannot be read" error which would never reverse itself.
Especially in comparison to a dedicated HTPC, the media capabilities are paltry.
Sorry, I tried my hardest to make the 360 usable as a media player - I even went and hung out with the fuppes guys for 3 weeks trying to get the video transcoding working - but it just doesn't cut the mustard.
I've never had any trouble streaming video the Xbox 360... its worked great... but of course I'm not using the "Media Center" connection... So maybe that's the difference.
Connect360 for the Mac has been great.