CNET does a comparison between the Apple TV 2 and the Roku 2:
There are more ways to stream Internet video–Netflix, Hulu, Amazon, and the like–than ever before. On that subject, I’ve laid out my hardware recommendations for a variety of consumers in an earlier story (Which streaming-media device is right for you?) But looming large in the marketplace these days are the two $99 offerings: Apple TV and Roku 2. It’s enough of a horse race that it’s worth a separate discussion.
I have experience with both of these boxes, and they each have their strengths. The Apple TV 2 is an absolute must have if you are already committed to iOS and the Apple ecosystem. The Airplay feature alone is worth the cost of entry, and when iOS5 comes out this fall look out for a console-style gaming explosion. The Roku works better as a standalone solution, and if you don’t have an iTunes library bursting with purchased video content it could be a better choice. The Roku offers “channels” that you can download and install for free- Pandora and Amazon being two of the more notable ones.