Firstly NOS is NitrousOxide, laughing gas!!!
Nitrogen is N2, in the UK we use compressed air (generally) tho there is little difference, although N2 is pure, so should be better.
Air is stored at up to 4500psi (300bar) in the UK, we dont really have access to 5000psi fills, which are more common in the USA.
Co2 is poo really.
You get many more shots from a tank of compressed air, than you do Co2 due to the storage preassure, unless I think you are using an old Shocker, they are better on Co2 I think (I know, I was surprised as well!!).
Low preassure chambers, reduce the preassure of each charge of gas/air, by increasing its volume...the idea is that a lower preassure charge, reduces the balls tendency to flex and wobble (like a balloon filled with water) on its way up the barrel, and through the air, thereby making it more accurate in flight.
Also reduces kick.