Typically I would say its something that depends on the type of viewing. If its something you are going to sit there and watch completely focused, or if its something that is on in the background or watch intermediately while socializing. But either way, I prefer it on my iPad.
I dont pay for cable, or even own a TV anymore, so thats the main reason. But something I ended up realizing going through various different TV screen sizes from a 15", to 32", to 46", to 55", is that once I am immersed in the content, the size is irrelevant. My focus becomes so much into it that I just dont even notice the size of the device it is on until something pulls me away. Once my focus is pulled away, I realize that the TV is small, or large. But at that point it doesnt even matter as my focus is elsewhere anyway. When I go back to focusing on the show, the rest of the world, including the TV size, seems to fade into irrelevance.
Even though that is the case, I can understand under some instances someone preferring a large 4k viewing experience for those movies with fantastic details. And while I admit it is gorgeous! Im perfectly happy watching 1080 or even 480 resolution content.
And I should make note that all of this is based on the presumption that you mean watching something alone. If you have 7 people trying to watch something off of an iPad, it would be a bit outlandish.