Re: AF Hookah "Light"
I would guess your assumption is correct, that they are hollow. Its probably just a brass core with the outside hollow designs soldered onto it somehow...similar to how a lot of KM stems are (i.e SSR; hollow and fairly light). Where are the 'solid' stems are just that; solid.
As far as smokeability....as long as the gauge/etc is the same, you will get identical smokes.
The 'solid' versus 'light' is probably more of a durability thing, but even then if you take care of your rig, shouldnt matter.