iS uses a non-adjustable gas shock coupled to a spring that is part of an adjustable pre-load system. It allows the ski to raise and lower the upper deck as necessary. What it does is that in smoother water, it keeps the deck down for a lower center of gravity and better handling. In water that is less smooth (I'm going to avoid using the word rough, in this case), it raises the deck to allow for more travel to keep the ride smooth. The key thing to understand here is that the damping provider by the damper is STATIC. The effective damping is changed because the ride height is varied, and thus it is changing the point in the damper's travel where ride height is. The additional plus about this is that there is a dock mode where the system drops the upper deck until it bottoms out. This is very good as it makes the ski more stable as it lowers the center of gravity by quite a bit (this matters more and more as you have more passengers/heavier passengers).
aS uses a Fox racing shock with adjustable damping (easily changed by a knob) as well as adjustable pre-load (but manual adjustments). The plus side here is that you change the damping and also, dial in much more damping than the iS system has. Damping is what you really need when you end up in really rough water and the ski is taking big hits (this is where the iS system will start to bottom out - and on that system, there's nothing you can do - once you hit the max, that's the max - on the aS system once you start bottoming out a lot, you can crank up damping a bit more to compensate). Because of this, this is better for very rough water than the iS system. The iS system is more of a touring system while the aS is designed for rough water performance and general harder riding. Of course, the downside to this system is that you would need to manually adjust pre-load any time your payload changed, otherwise the system won't perform as designed. Additionally, there is no dock mode, so the ski will perpetually have a less stable/tippy feeling at lower speeds.
I personally own a GTX Limited iS 260. I love the ski. I do very long destination style rides (sometimes 5+ hours a day) and I can't imagine doing rides like this on any other ski. It is very comfortable. But yes, in rougher water, I do reach the limits of the system and have to back off as I don't want to continually bottom out the system in repetition. In those cases, I wish there was a little more damping but I can't complain. Maybe when the stock shock wears out I'll see if I can find one that's adjustable. That would be nice. But for now, I just back off a little and it's fine.
