I have the WDTV Live media player. Although it has no storage (you can get an external hard drive), it plays all video formats (.mkv, avi, divx, m4v, mp4 etc.) and HD formats (720p/1080p), can playback movies with DTS soundtrack (where others don't), can connect wirelessly to the internet for YouTube playback among other things, can stream movies from any computer (PC or Mac) and it's really tiny.
Two drawbacks, though:
-You won't be able to get the menu/chapter listing option for m4v files (Cathe's workouts are this type) you get with iTunes. Only Apple TV has this option because it works with iTunes.
-It has no buttons on it at all so the remote control is always necessary.
I am not sure the new Apple TV can connect to an external hard drive. I think it can only stream movies from your computer.
I am thinking of buying the Apple TV just to use it with Workout Manager for Cathe's workouts only. I am using 2-4 DVDs on a weight day, and right now I am programming my 6-disc DVD player.
Chris.