Designing the piston in part design workbench is very easier than using GS, and as you told, this takes long less than designing it with absolute surfaces, initially. Nonetheless, GSD is very interesting and joyful because of its numerous techniques of shaping, though it is harder than part design. GSD allows us to design dreamy curvature things which sometime is not possible to do that in PD. But in the practical engineering works it depends on the requirements that which one is used.

As I have worked with solid parts in Auto Cad a lot, GSD is an interesting new workbench for me, while it has many additional facilities to design complex curvature objects.

However, PD and GSD each one has its particular technical usage and in some situations they are beside each other to create and industrial thing. So I can't opine which one is better; both are excellent in the circle of their particular tasks, nevertheless I mostly love GSD .