The lighter a blade is, generally, the less resistance it has to force or impact. If you're going for a lighter blade, it's to cut quickly and not bear any load. It's not good for a tangible swordfight, but it would function well for a fight with a saber fighter. Take the example of the Katana versus the Western Longsword. A Katana is traditionally a weapon intended to kill with one stroke, whereas the Longsword is geared toward longer, tougher fights. There's more blocking and parrying versus a fight between Samurai, which was generally either went to the first blow (thus the inception of Iaido) or favored the more skillful swordsman.
In a fight between knights, skill is a massive factor still, but armor is more important because they don't have that same emphasis on keenness of blade. It won't generally slice deep into armor and award a killing blow in one go. Halfswording, in fact, was born of the ineffective nature of a longsword in a battle involving armor. This allowed a swordsman to grip the blade of his weapon, sacrificing range in order to command greater bashing power or direct his strikes toward weak points in the plate.
In the end, it comes down to your preference. Some research required, of course.
