Visually, the MICH / ACH is a bit smaller than the PASGT, doesn't have the brim on the front (better vis), a bit shorter on the sides (better hearing/situational awareness), and shorter in the back back (compatibility with body armor, specially when prone). The MICH actually comes in three flavors, too: the TC-2000 (which was just described and is most commonly seen), the TC-2001 which has an open ear design, and TC-2002 which is a half-ear design.
Slightly less visible, the MICH has a four point retention system, versus the two point one on the PASGT.
And not visible, but most important, the MICH does away with the strap suspension system and introduces a pad system, not unlike what you'd find in a bike or similar helmet, which is better at absorbing shock and impacts (exactly like a bike helmet). The foam pads used for the replicas are different from the real ones, but they're still more comfortable than the suspension straps. And last but not least, while the PASGT is credited with stopped bullets it wasn't designed to; the MICH is ballistic rated.
PASGT
MICH/ACH
