I think you should take all the valid points, Alx9876 makes, but don't disregard the extent of the noise that comes from the 360.
Ultimately it was the noise level of the 360 that lead me to buy an AppleTV. Here is summary of the two as objective as possible:
Video:
360
Pros-Supports higher bitrates and resolution, DVD player included, better codec support
Cons-DivX does not work when not connected to Live
AppleTV
.Pros-High quality playback
.Cons-Flaky h.264 codec support. Poor codec support
Audio:
360
.Pros-5.1 via WMA and AC3
.Cons-AC3 implementation is a bit of a hack
AppleTV
.Pros-5.1 via AC3
.Cons-AC3 implementation is a bit of a hack
UI:
360
.Pros-Fast. Very good depending on UPNP streamer used. Allows custom categories like home movies.
.Cons-Slow and clunky depending on UPNP streamer used. only displays the title
AppleTV
.Pros-Very good with well tagged content. Cover Art and show descriptions
.Cons-Poor with badly tagged files. Only TV Shows and Movies categories
Online Store:
360
Pros-HD TV shows, Better HD Movie selection, Good TV selection, Good to Very Good quality
Cons-Does not sync content from 360 back to your MAC. Longer wait before ready to play.
AppleTV
.Pros-Syncs purchased content back to iTunes. SD shows are typically ready to play immediately. HD movies are ready sooner
.Cons-No HD TV shows. Typically lower quality, HD Movie selection is poor currently.