It's the expanded polystyrene inner that's going to save you (and potentially the MIPS from any twisting forces).
What injures you is the rapid deceleration/rapid twisting of your brain - your skull hitting the soft polystyrene means it decelerates slower than hitting a road/vehicle.
Obviously, an overly-weak outer shell cracking is a failure because the polystyrene is then exposed to whatever hit the helmet. The outer shell is mostly just there to keep the polystyrene in the right place - on your head.
Heavy? That increases momentum, and worsens any twisting force. You really don't want a heavy one.
You want something strong enough to resist falling say 2m, but light.
Buy a top quality one, look after it, and it will last until you actually need it in an accident. Replace it after a crash. Replace it when the manufacturer says replace it (5 years is typical) because the inner lining can become less effective as it gets gently bashed by your head after several years of use.
Last bit - make sure it fits well, and can't get dislodged easily. The best helmet in the world is wasted if it pops off your head 1/0th second after hitting the ground/vehicle.