The two metals are equal, but Netherium is 200 degrees C, and Blizzerium is negative 200 degrees C.
A later update for Chaos Theory will make them still identical besides that, but even more viable. If I remember right, they were based off of either iron or steel, but sharper. I might have changed the densities; I can't remember off-hand.
Either way, both metals are very very good when used as sharp weapons and still of high-quality when used as bashing weapons.
Fallens and Risens are made of Blizzerium and Netherium, respectively, but the ability for those metals to be cut isn't changed yet.
Any vanilla organic race with Netherium will have that part melt from the heat, but Blizzerium causes no negative effects, despite the fact it can flash-freeze water, and is quite close to Human Absolute Zero (Dwarven Absolute Zero is much, much colder, and much more dwarven).
So yeah, those two picks you got are weapons to be feared, but both Fallens and Risens have innate combat skills (Fallens have lower innate combat skills but have innate everything, while Risens are machines of pure murder. Their weak point is their heart, somewhat).
Funnily enough, pentahydras aren't injured by holding a netherium anything.
I'm assuming that Fallens hate Pentahydra due to ethics - Fallens have ethics similar to dwarves, but with some modifications, while Pentahydras have goblin ethics down to the letter. The reason will be explained when I upload the official Beta v1.1, which will include more things. The Pentahydra are the only new race, however.