I'd recommend a full face since they're dual purpose. Polycarbonate helmets offer good protection and are lightweight. Good composites take it up another level. Composites transfer more shock than poly's so they depend more on the styrene inner shell to minimize concussion/brain trauma. Composites offer better penetration resistance.
In other words -- some plastics are better than others.
Composite full face helmets will probably offer better chin/face protection as they should be less prone to collapse on impact with the steering wheel.
In other words -- don't smack your face on anything so that this won't be needed!
Good ventilation is important with any helmet, especially in a car in hot weather with no AC.
You'll sweat like a pig in a helmet no matter what!
Generally there are two shapes of skulls, round and egg shaped. It's critical to try helmets on before purchasing. They should be relatively tight with no pressure points and left to right horizontal rotation (losness) on your head should be no more than 11 to 1 on a clock dial.
In other words -- GO TO A STORE AND TRY THEM ON BEFORE BUYING -- THEY ALL FIT DIFFERENTLY!
Brand names typically draw premium but there are some equally protective "off-brands" on the market, sometimes the shells are made by the same company that supplies the brand name manufacturers.....at least that's the case with motorcycle helmets.
A tip here -- Helmets have to be rated and tested -- Look for a SA2010 rated helmet.
Also consider face shield options available, tinted shield are better than sunglasses and offer good eye protection.
Good advice because stuff has a habit of trying to find your face....
You want a liner and cheek pieces that's easy to remove for washing and most manufacturers offer multiple sizes of cheek piece to tailor the fitment.
Because of the above mentioned -- sweating like a pig in a helmet
The Bell Racing Sport helmet would be a good baseline to start from:
http://bellracing.com/helmets/pavement/autocross
The Simpson Bandit is another good helmet that's pretty light.
http://simpsonraceproducts.com/produ...id=10399&sort=
If you want to go big league Schuberth is a favorite in F1:
http://www.schuberth.com/en/