This is not an either/or question. Both enchantments have their own purpose. I disagree with this choice of Mojong, but it is what it is so onto the dilemma. When starting a new world, your goal should always be to get a perfect bow with Infinity. If you have Unbreaking III, you won't have to worry about repairs very often especially if you have a good sword. Once you've set up a skeleton spawner, consider your self to be somewhere around late-game. At this point you should definitely consider switching to mending, especially if you have crafted shulker boxes and enchanted arrows. This solution should provide a confident answer for anyone questioning the choice.