I have the 21" Airweight and the 26" regular weight. The Airweight is far less substantial than the heavier model, even accounting for its shorter length. Imagine holding a 16" piece of aluminum pipe and a 26" piece of steel pipe. The steel has a momentum when you swing it that cannot be equalled by the aluminum.
The Airweight requires a significantly harder snap to deploy solidly. The steel baton opens with a slight flick. A hard flick and you'll have to hit it hard against something to get it closed again. With the Airweight, you have to be careful to open it solidly, especially if you plan on poking with it. (Ok, I just tried opening the Airweight with a very hard snap, and it took me six solid blows on the floor to close it...and now my hand hurts.)
Now, for someone trained in the use of a baton (which I am not), I think that either one can be used effectively. I can't imagine hitting anyone a solid blow with the steel baton without doing serious damage. In fact, I think it would take a certain amount of training not to cripple or kill someone with it. Elbow, knee, or head blows would be devastating. With the Airweight, a hard whack could do considerable damage, but nowhere near what the steel one could.
As far as carry is concerned, it all depends on how you'll carry and whether you have to conceal it. You could slip the 16" Airweight in a pants pocket without a problem, although it might protrude a bit. Either one would fit in a winter coat pocket. For the 26", I think you'll need a belt holster. ASP makes shoulder holsters for them too. In a holster, I don't think you'd mind the added weight of the steel, although you might have to think about balancing your load if you carry guns and knives too.
If you're going to count on the ASP as a primary defensive weapon, I would suggest getting some training with it. I'm also assuming you know that carrying extendable batons is illegal in most states. You might also want to consider getting a good, stout, crook-topped cane. Not as convenient to carry, but very effective and raises no alarms from the populace.
LOL