According to my experience is just about the type of grip and the way of tattoing. With disposable tubes heavy machines tends to be unbalanced and the light ones for some artists tends to be not enough heavy for lining or coloring.
Personally the ligthes the better, cos i use almost entirely disposable grips. Expecially with magnum , when during a tattoo i need to turn the machine a lot of times i prefer light rotaries or alluminium coils, I noticed with eavy machine to tend to suffer wrist pain after ours of tattoing.