If you have an iPhone, iPod Touch or iPad, you can also airplay a lot of videos or audio to the Apple TV.
But the big question is what do you want this box to do?
Currently, the Roku has the advantage of having the most UK streaming services: BBC iPlayer, ITV Player, All 4, Demand 5 and Netflix.
The Apple TV only has Netflix, but the new Apple TV will get additional apps soon (BBC is in the pipeline, and the rest will follow eventually). Until then, All 4 and ITV Player will airplay across.
The new Apple TV can also easily add apps from different countries, so if you use Smart DNS, you can add TenPlay from Australia, PBS, Crackle NBC etc from the US and add additional ways to watch more shows. The Roku can not do this.
The older Apple TV can not do this either, and will likely never get more UK apps. But it is cheaper than the new model.