Don’t get a motorcycle or ATV helmet. It weighs A LOT, and is very hot (it is designed for ventilation at speeds you will not reach on your motorcycle).
I do a lot of mountain biking (all types, XC, Urban, Freeride, but mostly Downhill), trust me when I say you don’t want a motorcycle helmet, not even a motocross one. And most certainly not a full faced.
If you are serious about your head protection, get a fiberglass, kevlar, or CF full faced mountain bike helmet. I personally wear a Mace Hammer ( http://www.macegear.com/ ). I wear it because it fits, is light, and I know it is strong (I am alive today because of one that now hangs in my office). SixSixOne, Bell, Giro, Troy Lee Designs. There are no shortage of good full faced bicycle helmets out there. Find the one that fits you best, and wear it.
Any good mountain bike shop will carry an assortment. Just watch out for ones that use standard bicycle crash foam and plastic liner. These are simply XC helmets (which are just restyled road helmets) with chin bars added and some protection added to the back of your head. The increase in protection they provide isn’t nearly as good as a proper FR/DH full face, but they tend to cost as much.
If you don’t want a a chin bar, look at a protec “skateboard style” helmet. These are becoming far more common on the roads and trails because they aren’t as cumbersome as full faced, but provide excellent protection to the sides and rear of your head that a “normal” XC mountain bike helmet does not. In all honesty, unless you are pushing yourself hard enough that you fear smashing your face, this is probably the best helmet style to wear.
Trust me when I say, overly heavy poorly ventilated helmets can really suck the fun out of mountain biking.
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“The two seconds between ‘Oh S**!’ and the crash isn’t a lot of practice time.”
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