There is no doubt that the Apple ecosystem (iDevices with AppleTV) just works better even in it's current form. You can Airplay your iPhone, iPad, MacIntosh to your AppleTV or do mirroring (I do this daily and never had any issues with Airplay).
For Android users things like Amazon FireTV and NVidia Shield are their best answers, but the people here are Mac users.
The new Apple TV, or whatever it'll be called, will match the features of the next gen boxes that just came out and extend your Apple ecosystem far beyond the other set top boxes. Plus the ease of use and the huge apps development will eclipse all other competition. Apple TV for me works fantastic without customizing some add-ons from another set top box maker to get a few extra capabilities that will come under the next Apple TV anyway.
With the console-like games development and quality support coming from Amazon and NVidia, I'd expect Roku to be the bigger loser in sales against those two and against the new Apple TV. Currently Roku 2 has less features, power and less available games. Roku has the largest marketshare now at 34%, but it is in a poor position with less strategic resources compared to Amazon and Apple.