Weight (mass) has nothing to do with the rate or speed at which an object falls.
Actually it does have something to do with it. I mean, in theory, if there is no sort of drag, every single object will fall with an acceleration of 9.8 m/s per second, that's what we learn at school. However, in real life where drag exists...
Very true. I think sometimes its overlooked that objects will tend to fall at 9.8m/s2, but only in a vacuum will other factors like friction and weight not apply. For example in this case weight does have something to do with it, as the heavier the car and the less friction between the track and the car the easier its going to make it for the object to reach 9.8m/s2.
So in this case I would say the main difference is in restraints, the weight, and most of all design, for example how the cars are attached to the track.
In the First Gens all you had were free spinning wheels, with a slot that literally opened up and let the cars fall. There was literally nothing to slow down the initial "drop" and I think that's why most people when talking about Intamin 1st gens say they get more airtime than any other kind of drop. And I have to say I agree, 1st gen drops definitely do a better job of giving that natural sense of freefall.
On the newer gen towers the cars feel like they have a much tighter grip on the track, not to mention contact with the magnetic track (which controls its stop) all the way from top to bottom. And even though it seems like such a slight difference I think in the end it makes a somewhat noticeable difference as most of us can agree after ridng Lex Luther that the drop is not nearly as intense.