Lots of great stuff on this thread already, but I'd like to add that used to think that Bell helmets were more comfortable, and started wearing a Giro Atmos because it fit better. I didn't think it was more comfortable, but it sat on my head better, and was more secure.

Then I fell down and went "boom". Had to switch back to my Bell Alchera. Yuck, I missed my Atmos.

Helmets are personal things. Get waht works/fits for you. I think the Bell/Giro conglomerate do a good job, and I know my family appreciates the time they have spent designing their helmets.