Every head is different.
Some folks say they have a "Giro head" and those fit best for them.
Others have a "Bell head" and like them.

It is best to try them on and see which one fits you best (and appeals to your sense of style!).

That said...I have a Bell something-or-other. I had a Giro Pneumo but it broke (non-accident) and I didn't like it anyway, cuz it cut into my forehead and left me with a big ugly bump all the time. The Bell makes me look like a big mushroom head, but it's very comfortable.