It's not just for cutting, almost every part of the weapon is good for something. The katana is a specialized weapon, excellent for cutting, whereas the the longsword is more of a multi-purpose tool. The longsword can be used to preform all sorts of attacks and defenses, making it an effective weapon against all sorts of enemies. It also looks better, but that's just personal preference.
Yeah... That's pretty much it.
-Racob has spoken