I think, if anything, soldiers that use two handed swords should become fatiqued more quickly. Sure, you can swing a greatsword pretty quickly the first time, but try swinging it about 50 times and check how you feel. I used to practice with different swords and axes and I couldn't fight with a greatsword very long(course, I'm a wee lil' man). There are other variables that weight affects besides just speed of your attack. It also affects accuracy. A large sword is alot harder to 'aim' mid-swing compared to a lighter Rapier or Katana. Its takes alot of energy to change the direction of that much mass once its been swung.

Well, a Katana is a unique story, since it has the weight of a longsword but is used in a two-handed style, which is the basis of its amazing abilities. The one sided nature allows it to be longer while maintaining similar strength charecteristics to a sword of its weight. The two handed wielding style reduces fatigue, allowing you to fight longer, and the light weight paired with the strength of two arms makes for very accurate cuts. Yeah, i'm a big fan of katanas (and other Japanese weaponry).

The advantage of the Zweihander is that its got a grip that goes beyond the guard. Gripping the sword by that area makes the sword move more quickly. Fighting with a Zweihander is really technical, because you switch between gripping it by the grip and by the flat edged portion of the blade, depending on what sort of attack you wanted to do. They also typically come with a enlarged hilt for counterweight effects, and as a blunt weapon that you could use when gripping the flat-edge. The whole point of giving it a flat edge holding area was to make the sword versatile, and capable of handling a variety of situations.

I still like the Katana more though.