Large eyeport helmets are preferred by sedan racers because they make it easier to see the gauges. Full-face helmets without shields have even better visibility and are much cooler, but do not completely protect your face. Open-face helmets offer the best visibility and comfort, but the least protection. Some helmets have vents, some don't. More vents may keep you cooler, but they also provide more ways for fire to get in.
Most helmets are fiberglass.
Helmets are also made more expensive by aerodynamic engineering to prevent lift and buffeting in open-cockpit cars. Correct helmet fit is critical. A helmet should be as snug as possible without giving you a headache. You can start with your hat size but that is just the beginning. With the helmet on, chin strap undone and neck held rigid, twist the helmet left and right to check for excessive play.