I own both Roku and Apple TV. I love them both, but for different reasons:

Things that come to mind the most for both products:

Roku

• so many apps and new ones arriving constantly
• stream videos from your computer to the Roku through myplex or a hard drive depending on the model
• no YouTube app, which is a huge downside for me
• international TV channel apps

Apple TV

• AirPlay, but not all apps support it
• stream effortlessly from your iTunes library
• a YouTube app, but it's missing features like recent list of videos from subscribed channels
• Trailers app really nice

I like my Roku better. The Apple TV for Apple is a hobby, we might see it take off once Apple's television comes out in a few years.