The answer would also depend on the features of a smart TV and an Apple TV you would want to use. If you have other Apple products the Apple TV will integrate well with them, especially an iPad or iPhone, but a smart TV not so well if at all. There are also features on a smart TV that are not on the Apple TV, although they maybe possible with a iPad or iPhone using AirPlay.
I'm also not impressed by the user interface of these apps on the smart TVs. It's not really the strong point of TV manufacturers. A friend just bought a new Sony TV. I'd be more inclined to use the equivalent features (like YouTube) on the Apple TV than the apps on the Sony. Since I have an iPad I also get a better experience using my iPad with AirPlay than a couple of the other apps on he TV.