Difference in MICH2000 and ACH

I have two helmets. One is issued, and one is more comfortable and personally owned. I'm wondering what the difference is as they have two separate NSN numbers. Both numbers look up to be basically the same thing:
http://www.armyproperty.com/nsn/8470-01-506-6373
http://www.armyproperty.com/nsn/8470-01-529-6329
http://www.armyproperty.com/nsn/8470-01-506-6373
http://www.armyproperty.com/nsn/8470-01-529-6329