I wanted to chime in here...
The US Navy and NOAA have done more for diving then any other group out there.
While the US Navy was very propritary in their equipment years ago, they opened their minds to the developments from the commercial diving industry. The sharing of technology and theory has only served to make diving better and safer for everyone.
On the point that the Poseidon or AGA are used by the military or commercial divers for deep diving,,,both are considered light diving equipment like the old Jack Brown mask was and are not used for deep diving. For deep diving, the standard has been and is the DSI Superlite (17 or 27) helmet. When lighter gear is needed or preferred, the DSI 18 Band Mask is generally the chosen tool. The AGA and now the Poseidon (given their similar lineage) are predominatly used by Police and similar organizations for what would be considered blackwater and cold water diving for the enhanced protection over conventional Scuba. (Honestly, when I dived in muck, I preferred an AH3 airhat locked into a Viking drysuit).
Bottomline, when it comes to full face masks or helmets, the US Navy and the commercial diving industry wrote the book. To ignore their influence and contributions as a protest is unsafe.
As far as the Neptune, if it floats your boat, go for it, but recognize it's limitations. The AGA is beyond proven and the Poseidon (even though it is new) is a refinement of the AGA with a better proven 2nd stage and improved design features.