A heavy blade takes more power and time to reach speed but once there, hit a patch of tall tough grass and it maintains its speed better than a light one. So it's a washout.
If it hits a rock, a heavy one could get through or over it where a light one would shatter. Or a light could get stopped with just a nick, where a heavy would be wrecked because of the greater inertia. Again, a washout.
As for materials, to make a long story short I had to make a set of blades myself. I couldn't easily get or work blade-grade stock so I used ordinary steel. They have performed and lasted just as well as factory-made.