Both systems are good as far as getting the basic job done, but I'd rate the AIR over the Arm.
The convenience of the AIR enabling you to quickly & accurately dial in the output pressure you require is its biggest plus point. In addition you have awesome support from WDP & the hoards of certified techs plus my impression of having used both systems is that the AIR is more consistent. As an extra bonus - which may or may not mean anythign to you - the dual safety standards (HSE & DOT) nature of the standard bottle means that you can legally play with the system in any country until it requires it's first retesting.
I also believe that the Armageddon has a couple of issues (I'd call them faults but they may not bother other people so much): removing the bottle from the reg whilst dialed in to a high output pressure can be quite difficult due to the air pressure resistance & tends to result in lots of aluminium shavings being scraped off the reg's threads - there is a possibility, albeit remote, that one of these shavings could be worked into the reg whilst removing/attaching the bottle; the bottle has to have part of the system glued into the bottle to ensure that it does come undone during normal usage of attaching/removing the bottle from the reg, thus you'll probably have to send the bottle away to get this part removed when you eventually need to get the bottle retested.
