MICH and ACH are basically the same thing. MICH is Modular Integrated Communications Helmet. ACH is Advanced Combat Helmet. MICH is the system, ACH is basically the helmet portion of the system. Both terms are interchangeable.
I'm not sure what you mean by MAS. MSA is a brand that makes ACH helmets. As does Gentex and BAE.
There are also 3 basic styles of MICH/ACH helmets. 2000, 2001, and 20002. Often prefaced by "TC" (eg: TC-2001)
(Left to right) 2000 is standard, 2002 is mid cut, and 2001 is high cut
ACH helmets use pads to fit, as opposed to a suspension system. Used standard ACHs in the EE run about $150-200.
The USMC LWH (light weight helmet) is basically a lightened PASGT. There are accessories out there for it, although it's not as nice as the MICH setup.
I'm not sure what your budget is. There are also RBR and Kejo helmets. These run about $350 new, and if you want a high cut helmet are less expensive than similar high cut ACH helmets, which run about $500 new. Although, they don't have an ACH headstrap bolt pattern or size, so they won't fit some accessories like rail systems. I also find the suspension systems less comfortable. Although you can retrofit a RBR or Kejo with pads and a helmet strap.
If you have some real money, you could also check out the OPs-Core FAST Ballistic or a Crye Precision Air Frame. Although, those will run about $950.
OP, what's your budget? Do you have any specific needs? Maybe that will help us point you in a right direction.
ETA: There are usually some good deals on helmet setups in the EE
Medium TC2000 with Norotos Shroud and Arm
Large MSA TC2002 fully kitted out
Large Gentex ACH $250
Medium ACH -$150
Standard ACHs are the best deals going right now.