Had the 'pleasure' of both types- even fitted properly on my 7 3/8 brain housing unit, the M1icon did not sit deep enough to prevent the bobblehead effect, which we never realized until the kevlar came out. We just got used to it. The 'kevlar' sits lower and thus lowers the CG. If you add a helmet pad, the M1 is even worse, the kevlar less so. I do admit that the kevlar could also (when you hunched down in a sleep position) rest on the older flak vest and thus let you catch a few Zs. But then I never had to use my M1 to shave or boil water with, went to DS2 with a kevlar. Since then, I became an 8th grade teacher and have examples of both in my class for when we get to VN and post-VN in the social studies curriculum.