In order to reach high alcohol concentrations (ABVs) such as >75% ABV, stills typically require a column filled with saddles (like the T500) or bubble plates like other commercial and homemade distillation equipment. This is done to facilitate refluxing action inside the column.
As the wash boils in the boiler, a vapour containing both alcohol and water is formed. Before the vapour reaches the condenser it refluxes in the column part of the still. Refluxing enables the water to be stripped from the alcohol as the heavier/less volatile water molecules in the vapour condenses on the saddles, leaving the more volatile alcohol vapours to move up the column and into the condenser. It's because of this separation due to refluxing that such high ABV's can be achieved. It is also because of this refluxing action that a consistent ABV is achieved throughout the distillation run from reflux stills (such as the T500).
The Air Still, on the other hand, is closer to a pot still configuration rather than a reflux still. It does not contain a column filled with packing and the output ABV starts high (approx 65%) and decreases throughout the run. This is because there is no (or very little) refluxing in the unit. The alcohol/water vapour formed from boiling the wash makes its way directly into the condenser and condenses into liquid alcohol without undergoing a secondary separation process i.e without refluxing. Before reading onwards please note that ethanol has a boiling boint of ~78.3°C (~173°F)and water has a boiling point of 100°C (212°F).
At the beginning of the distillation on a pot still, there is more alcohol in the wash so the wash boils at a lower temperature (closer to 78°C (172.4°)). This means that the majority of the vapour being formed will be alcohol with a little amount of water due to the relationship between water and alcohol when it is boiled (search azeotropes for more info). As the distillation progresses the boiling point of the wash will slowly increase and move towards 100°C (212°F). This is because the alcohol content becomes lesser and lesser in the wash as the distillation progresses. As the wash is now boiling at a higher temperature, more water vapour makes its way into the condenser so the resulting distillate coming out will have a lower ABV. In summary, without refluxing, the ABV of the distillate being collected will generally be lower and will not be consistent throughout the distillation (ie starts high and ends low).
So why have pot stills you may be asking? Well, the major benefit of a pot still is that the distillate collected retains the flavour of what is being distilled. In a reflux still, all flavours are stripped from the vapour and only the most volatile alcohol is condensed and collected. On the other hand, in a pot still, the entire vapour generated in the boiler is condensed and collected. So if you want to make whiskey from a grain wash, or rum from molasses - use a pot still.
The Air Still, even though it is closer to a pot still, can still be used to distil a sugar wash to get clean alcohol, however, the quality of the neutral spirit will never be as good as the T500 nor would it be at such a high strength (even if the alcohol is re-distilled because of the reasons mentioned above). What the air still offers is the ability to make neutral spirits using a counter-top sized unit without using cooling water. It's definitely a great piece of kit to start the distilling hobby.