Fit is Paramount for the impact. Weight is also important for muscle and skeletal damage from your head swinging around in some tumbles (this is the #1 consideration in most SA rated auto sports helmets IMO). This is were a lighter Arai or Suomy may sacrifice a very small impact protection factor to achieve a lighter unit maybe? Another important impact feature is the efficiency of the energy absorbing foam and how it distributes the forces into the softer and harder spots in your skull, not just the impact resistance of the shell. Don't crash test your helmets and none of this will matter! When I researched the common serious injuries that motorcycle racers suffered in crashes, head trauma was WAY down the list, so look into other saftey gear as well.
P.S. The Arai Corsair V is the most noisey, hollow sounding helmet I have ever used (also the lightest). Good luck!