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I'd be more concerned with how different the helmets are.
A pro-tec is a soft foam helmet which means its only going to keep your head safe to a certain point (don't hit and trees).
A Holt on the other hand is a hard foam helmet, this can work for or against you, off of most jumps if you slam your head in one of these the hardness of the foam in the helmet will most likely give you a concussion (better then not wearing a helmet obviously). However it will save your head from being split in half while hitting a tree.
Honestly. My friend has the Giro Shiv. He says it's AMAZING, how comfortable it is with goggles underneath the helmet AND on the strap.
But other than that, Holt is good. But I still recommend the shiv.
try both on
make sure you get certified ones because i know there are some non-certified pro-tec ones out there. just get whatever you think fits you better, they are both dope helmets. i personally have a holt