What is the difference between these two styles of helmet? Most "serious" cyclists (e.g. in spandex with clipless pedals) seem to have the first type, whereas I've seen more "casual" cyclists (e.g. on single speeds, cruisers) hanging around Brooklyn with the latter type. Is there an advantage to either? (in safety, performance, etc.)
During longer periods of fast road cycling, the venting in the first helmet will make more and more of a difference. It's impossible to lose trapped heat otherwise and then you'll have sweat pouring down your face..
If you're commuting then the shell helmet is fine.