It seems it's mostly a downgrade from the technical point of view but a gain in lightness and maybe resistance
I'm using this two wooekipedia articles to make a comparison, they are based mostly on the visual guides.
The first order helmets are lighter but in exchange the lack the advanced imaging gear it's imperial predecessor had. The imperial helmet featured built-in filtration systems that extracted breathable atmosphere from polluted environments while the first order can't filter toxins.
The material is different since the new helmet and armor are made from betaplast instead of plastoid. How the two materials compare is yet to be fully known but we do know that:
Captain Phasma claims that "even a Wookiee can't crush First Order armor", which seems like a considerable improvement given that imperial armor could hardly stand against the ewok's primitive weapons.
The new material is fire resistant,something we know the original plastoid was not (consider Vader helmet charring)