Both Bell and Simpson make fine safety equipment so you can't go wrong with either. Imapact Racing (Bill Simpson's new company), Pyrotect (an off-shoot of Bell), Arai, OMP, Sparco and G-Force are also good brands to consider.
As others have mentioned, fitament is the most important thing. If you can try some friends helmets, or find a local shop to test fitament before you buy, that's probably the best option. If you don't have that option you might look here:
http://www.saferacer.com/autrachel.html
I've dealth with that company several times and they are good to work with. They have nice fitting charts so you can always pick one you like, order it and and return it if the fitament is off. I have no affiliation with that company, yada..yada...
Also, as far as ratings, the most important thing is to make sure you get an "SA" rated helmet, not an "M" rated one. Some DE's and most autocrosses will let you run with an "M" helmet, PCA events require the "SA" designation. The "SA" helmets are more expensive, but specifically designed for an automotive application.
Good luck.