I am going to jump in on this convo because I am looking at these also.
Can Apple TV do Amazon Prime videos?
No and it probably won't happen until/unless Apple and Amazon make up. Right now, they are very much at war and each is more likely to be proprietarily locked from the other as not.
Apple, unlike the Roku, doesn't allow you to add channels either. Personally, I think the Apple TV works better than the Roku at what it does but the Roku does much more. Apple TV's current channels are Netflix, Hulu Plus, MLB.TV, NBA, NHL, Trailers, You Tube, Vimeo, Podcasts, Radio, Photo Stream, Flickr and WSJ Live. That, plus full integration with Itunes is all you get. Roku claims over 600 channels IIRC although many of them are novelties at best for the average viewer.
Steve