I have apple tv2 and 2 roku Lt boxes in my home. I like the interface of Apple tv2 better than Roku and the movies are $1.00 less expensive to rent than on Xfinity, but occasionally you get some buffering which is annoying. I suggest hardwiring it if you can. Roku lets you get Amazon movies, so if you are a member or a prime member, it is better.
Mostly I use it for netflix, which Roku does as well. Roku does Hulu+, which my wife uses for the old tv shows and many other channels ( Apple tv does too) .
Roku is now $49 ( wireless only) for the LT 720p version, of which I have 2.
If you care about airplay and mirroring, the Appletv is best. If not, which I don't, you can get 2 Roku boxes for price of 1 Apple tv2.
All in alll, if all you care about is streaming, go with the cheapest device. If you care about integrating it into your Apple ecosystem of devices, then Apple tv is best.