No comparison whatsoever Im afraid. Even as a Windows Media Center extender, while its a nice interface, the whole experience is dependant on network quality and your remote PC being "awake".
Apple TV for interface is the best I have used, and believe me Ive gone through many media devices, (XBOX, XBOX 360, PS3, WDTv) and the Apple for me, offers the best media experience, be it music, movies or photos.
A friend has a Sonos, and for music that is truly awesome, especially when using an iPad as a remote.