There was an interesting article in one of the motorcycle magazines (Motorcyclist) a year or two back wherein they compared the cheaper helmets against the more expensive brands, and actually did some instrumented testing along the lines of what the regulations require.
The bottom line was that there are different, conflicting regulations and helmets have differing constructions to meet either set. This has implications for the design and performance of the helmet and if the type of impact specified in the regulations is not likely in your intended use, a helmet that protects "better" might actually do you some harm.
Some of the cheaper helmets were rated by them as better for a motorcyclist, based upon the type of impact likely to happen to a rider. The article is well worth reading and discusses this in great detail. You'd have to think about what your risks are, in a car, and decide what helmet standard most likely is best to use as a measure of performance.
The article is available online here:
http://www.motorcyclistonline.com/gearbox/motorcycle_helmet_review/index.html
Be safe,
JR