Fit is definately top priority. I was told at a small Arai only seller that fiberglass is super fantastic for saving your skull, which I immediately considered to be made-up as I've worked with fiberglass before when doing stuff with boats, and its not some miracle material. I thought I'd look it up and I found a really long article that I pulled this excerpt out of:
"The stiffest helmets in the Big Drop test, the Arai Tracker GTs, hit our hypothetical head with an average of 243 peak Gs. The softest helmets, the Z1R ZRP-1s, bonked the noggin with an average of 176 peak Gs. This is a classic comparison of a stiff, fiberglass, Snell-rated helmet, the Arai, against a softer, polycarbonate-shell, DOT-only helmet, the Z1R. OK. So let's agree that we want to subject our heads to the minimum possible G force. Should we pick an impressive, expensive fiberglass/Kevlar/unobtanium-fiber helmet—or one of those less-expensive plastic-shelled helmets?
Conventional helmet-biz wisdom says fiberglass construction is somehow better at absorbing energy than plastic—something about the energy of the crash being used up in delaminating the shell. And that a stiffer shell lets a designer use softer foam inside—which might absorb energy better.
Our results showed the exact opposite—that plastic-shelled helmets actually performed better than fiberglass. In our big 3-meter hit—the high-energy kind of bash one might expect would show the supposed weaknesses of a plastic shell—the plastic helmets transferred an average of 20 fewer Gs compared with their fiberglass brothers, which were presumably designed by the same engineers to meet the same standards, and built in the same factories by the same people.
Why is this? We're guessing—but it's a really good guess: The EPS liner inside the shell is better at absorbing energy than the shell. The polycarbonate shells flex rather than crush and delaminate, and this flexing, far from being a problem, actually lets the EPS do more of its job of energy absorption while transferring less energy to the head.
Remember, these polycarbonate helmets from both Icon and Scorpion are also Snell M2000 rated. So they are tested to some very extreme energy levels. And Ed Becker, executive director of the Snell Foundation, is on record as saying that a low-priced—that is, plastic-shelled—Snell-certified helmet is just as good at protecting your head as a high-priced—that is, fiberglass—Snell-certified helmet. So at the high end of impact energy, we have the Snell Foundation vouching for their performance. And our testing, without the extreme two-hit hemi test, says they're actually superior. "
whole thing here:
http://www.motorcyclistonline.com/gearb ... et_review/