Here is a like to Webbike world with info about head shape and helmit fit and also discusses the article about helmet composit response to an impact
http://www.webbikeworld.com/motorcyc...helmet-faq.htm
Most important above anything is fit, fit, fit - your helmet has to be comfortable to wear.
Go to your local dealer and find someone with experience fitting helmets and try on several different brands and types within brands. Don 't just do an on and off - let the lid sit on your head for 10 minutes or so to allow the padding to settle at least a little and look for any type of hot spot ie pressure point - if the helmet has a pressure point that you notice move on to the next one - once you find two or three that are in the running try them on again - choose the one that fits best
Then reward the guy who has just spent 2 hours helping you and buy the helmet from the shop or have them order the one you want - You can do all this and then go online and buy the helmet and save a few bucks - but IMHO the time and advice you just garnered about this most valuable piece of kit. As an engineer my 2Cents about the whole motorcyclist article is that for the most common accident your average road rider is likely to encounter say a 50 mph laydown is better protected by the softer non-snell helmets due to the energy absorbtion abilities of the material used in non-snell helmets. However, take that same 50 mph crash and go lid first into a blunt object (curb, fender, pole etc) now you have an entirely different type and direction of force being applied to your lid - here the Arais and Shoeis and other snell appoved helmets are better equipped to save your noggin.
Back to my original statement - fit, fit comfort comfort - Chose the design that you absoluetly can' t live without - purchase and ride like you stole it Long winded but I hope it helps