Awhile ago I found this review of a consumer reports review
http://www.helmets.org/cu_2006.htm

I never thought of helmets being safer than each other, but I guess consumer reports says there are differences.
They like the Bell Metro.
I tried it on - very uncomfortable. I ended up with the Specialized that consumer reports also listed as a good helmet.
I also have a Louis Garneou.
The Bell/Giros are all more round than my head and therefore uncomfortable.