I get asked this question a lot. You can find comparisons of these products here and here. Or, just know this: both are excellent products but the Roku 3 (or Roku 2) offers more channels and costs less than the Apple TV. If you already buy and view content ( TV shows, movies, etc.) from iTunes, or you want to view content from your Apple computer on your TV, then the Apple TV is your product. Otherwise, Roku is the way to go. Both products support Netflix, Hulu but only the Roku supports streaming from Amazon, and Amazon is now providing original content. It’s first show is called Alpha House.
Summary: Choose Apple TV if you buy from iTunes or want to stream content from your Apple computer to your TV. Choose Roku if you want the most viewing options including Amazon and/or you want to spend a little less money.