1) Ninja is shorter due to some internal passage differences
2) Ninja reg piston is brass (light!) vs. PE reg's stainless steel (heavy)
3) Ninja reg uses a coiled spring vs. the PE reg's belleville stack.
4) Ninja bonnet is slightly shorter and has a shorter pin valve.
5) Ninja spring cavity is slightly shallower, which precompresses the spring as the bonnet is installed(hence the need for the bonnet tool) So you'd have to add a couple red shims if you wanted to "Ninjafy" your PE reg by buying the Ninja piston, spring, bonnet and pin valve.
I've put the PE together with Ninja internals and the Ninja together with PE internals and there doesn't seem to be any reason why either configuration wouldn't work. Heck, the bonnets seem to be interchangeable. Same dimensions, same threads.
I don't have a set of small pin gauges, but I suspect even the base hole (through which the tank air is metered into the pressure-controlled volume, and on which the seal on the small end of the piston rests) is the same diameter. This would make the pressure ratio the same, and both regs ought to have similar in:out ratios, which they do.
Makes me wonder if the PE reg's dismal creep performance was due to a crappy piston base seal.
Edit: One huge plus on the Ninja side is the really long set of threads going into the tank. The PE engages about .6" worth of threads while the Ninja extends nearly a full inch. More threads = more safety.
This post has been edited by MondoMor: 21 August 2009 - 08:49 PM