Hp Splitting

You are at war and have 2 identical stacks of bombers (amount in stack, research level). One takes damage and is now 50% hp while the other if full health. In what situations is it more useful to land them at the same airport and split them evenly to 2 stacks at 75% hp, and which situations to leave a full healthy stack and the one at 50%.

Also, what would the maths look like if both of these situations attacked the same stack and how large would the difference be?