We get this question a lot, but it really depends on what you want and what you already have. I own an Apple TV 3 because I have an iPhone and iPad and it syncs nicely with both of them. I can actually share screens and remotely control the Apple TV with my iPhone too.
I have a Roku 2 XS and love it. There's not nearly as much free content on Apple TV as there is with Roku. There are tons of great channels, movies, TV shows, and access to other media that you won't get with a Apple TV.