I was looking at this one too, the only problem with it is that it's not as good as the ARB twin, I just found some sites where I can buy just the ARB compressor for less than $500.
Not sure what you are comparing to get the arb compressor to be "better," but I highly doubt that it is. I can't find the hp rating of the arb twin pumps, but you should remember that hp is directly related to pressure*flow rate. I'm skeptical that the arb twin has a true 100% duty rating, but I've never tested it out. I know those who have used the magnum air know it is a sweet little unit, just expensive. But, if it was between the magnum and the arb, i'd spend a few extra bucks and get the known better pump. JMO.
I spent the extra bucks to have it mounted outside of the cab, which with young kids and all my wife's crap, it makes it easy to access and doesn't startle the kids as much. If I was to get something mounted inside, I would get a puma.