You can find much more safety research on helmets than much of the protective gear out there. Just have to look.
Something nice about shopping in person...was at Kames today and was looking at helmets while thinking about this thread...most if not all of the high end helmets explain their research conclusions that lead to the creation of their helmet engineering logic in the product manuals. You can also ask the counter guys to look at the product catalogs of say Fox or Fly or Shoei and it usually explains some basics of their technological choices.
I would always suggest a Snell approved helmet at the least for adults...most kids helmet's are not Snell approved.
When I was a salesman at Kames...one of the things I would ask folks when looking to buy protective gear is..."What is the most important peice of protective gear?" As most would expect...the common answer would be the helmet.
No...it's your mind...making good decisions as a rider is most beneficial to your health and longevity in the sport as well to the other riders around you.
Then the helmet is after the mind, followed by goggles, then boots, then chest protector.
You can find vast amounts of research and testing on helmet design and function.
When selling neck braces...people would ask which one would I suggest...I would give them my personal opinions on pros and cons of each one an pros and cons of wearing one within the different designs or not wearing one at all, and then instruct them to make their own choice. I would also give them the facts. After that I would strongly encourage them to go do their own research and make a personal choice.
On many occasions...folks that were ready to buy that day would go back home, research and them come back to purchase if they did choose one.
As with any protective gear..proper fitmet is the very first priority and then the quality of the product.
As a salesman...it is dutiful to not oversell or undersell, but meet the customer right where they are at. But that also takes having good insight to the sport and knowing the individual customer.
Personal, face to face experience is what I love most about going to local shops. Kames is a leader in Ohio MX apparel and protective gear as for person help and counsel.