I think you are little bit off with standard dimensions: 120 - 48″ x 24″ x 24″ and 125 - 72″ x 18″ x 21″
Not only 125 has more footprint and territory for fish, it's easier to work with. 120 is tall and wide, making it harder to reach the far bottom. In term of showing fish, it depends on what fish you keep and where the tank is located. 120 is better for showing tall fish like angels and discuss, and if the viewpoint of the tank from both front and end.