One of the guys at my LBS mentioned the other day that Giro and Bell helmets are actually made by the same company. In my opinion, since all the helmets on the market are certified by the same organization, it's more important that the helmet fit properly and that you wear it corrently on your head. And if you take a fall at all and hit your head, be sure to replace your helmet, even if it otherwise looks okay.

Just my two cents.