I simply choosed Massamune because it's the best legendary sword. Excalibur has more attack power, but it's holy-elemental. Non-elemental is a lot better than holy-elemental since some monsters are immune to or absorb holy-elemental dammage. Thus Massamune is, even though it has lower attack power, still better than Excalibur in overall offense. And the fact that you always gets the first strike makes it even better.