I would choose the weight of the flywheel based on how much of the powerband you want to use. If your optimal powerband is 3-7K RPM, you would use a lighter flywheel than if it was 4-6K RPM. Since it is not cost-effective to change really adjust your gear ratios, those do not become a factor.
So if I was running an N/A or SC V6 setup with a broad powerband I would probably upgrade to a lightened flywheel. But if I was running a car that did not have a very flat powerband, I would probably stick with the stock flywheel since it would probably keep me in the meat of my powerband better than a lightened flywheel. Like a turbocharged four banger.
Maybe my reasoning is off, but that is my reasoning.
