If that empty tank is an AL80 at 99' it's got 1.6CF of air, of which 1.2CF is theoretically available by the time you reach the surface. Based on breathing a tank dry in 3 or 4' of water I'm pretty sure my reg would let me use a good bit of that 1.2CF during the ascent, so I'm not sure it's accurate to say the spare air has substantially more, especially since they still offer a model that's only 1.7CF.
Others may feel differently, but in addition to the other advantages of a real pony bottle I'd prefer to avoid doing business with a company that
1. Markets a nitrox version. Is that anything but an attempt to reel in people stupid enough to think the additional N2 uptake is going to matter during an emergency ascent?
2. Claims that spare air is better than a pony. Again, I see that as disingenuous and self-serving attempt to market to, and at the expense of, the stupid.
Sure, it's better than nothing. It's equivalent to a whopping 115psi in an AL80.
And as far as how to get it filled, that last means that using 3000psi rental tank will leave you with only 2885 psi for your first dive. Just suck it up and shorten your dive by a couple of minutes. Of course if you figure the spare air is good enough as a safety device you can be back on the boat with 385 psi instead of 500.