It's a toss up. If you've already got a bunch of apple shit, then the AppleTV rocks for stuff like AirPlay, pulling up your photo stream, pulling in your music library from iTunes Match.
If you don't have any apple stuff, then go with the Roku. Either way...wait for the updates that they'll both have for the holidays.
Since iOS is now 64bit there could be some wild stuff coming to the AppleTV soon.