XBMC would be for running on a dedicated PC that's hooked up to your TV.
Roku is meant to take the place of that dedicated PC and instead be a low-power, small box that runs Android and you can install apps and such. This includes watching media on it. Similar to a Chromecast, tho doesn't require a device to "cast" to it like CC does. Amazon also has their FireTV and are going to be releasing their Fire Stick soon (similar to the Roku).