I think Airplay is better for music because you have the controller in your hand and do not even need to have your TV on if you have an iOS device. MOG is built right into the ROKU interface, but it does not have the radio slider (it allows you to listen to one artist or blend in more similar artists as you slide it to the right on an iOS device).
Amazon and Plex are the main reasons to buy a Roku. However, you can jailbreak an AppleTV2 and install XBMC for these features. I have a Roku and jailbroken AppleTV2s and use the AppleTV2s quite a bit more (even before adding XBMC). The podcast feature that allows you to save favorites is really a nice feature on the AppleTV and you don't need a computer to be on to stream Youtube.
If you don't own a recent iOS device or a Mac with mirroring, I think the Roku is a better device. However, if you own a recent iOS device, I prefer AppleTV.
As I mentioned in another thread, I had issues with Plex on Roku, so I am reinstalling it on my computer tonight and will reinstall the app on my Roku this weekend. On the Roku, I was unable to play ALAC files the last time around. I am not sure if this is a shortcoming of the Roku app or Plex or if it was an issue with my setup, but it was a major downfall for listening to my music.
The upside is that my Roku does 1080p and my AppleTV2 is 720p. However, once my receiver scales the signal up, it looks very similar to the 1080p signal from the Roku. I do look forward to the update to Roku's interface. It is very tedious to have everything on one line.