I think the stock flywheel is 19 lbs, with the lighter ones being 16 lbs.
The heavier flywheel will help with stop and go traffic. But 3 lbs is not much difference. Some cars are able to knock over 10 lbs off their flywheel weight.
Supercharger? N2O or rear mount turbo seem like better choices. But they all have their advantages. I chose the cheapest of the 3, N20 for $1000. And that includes tuning and all the safety switches. But if $ wasn't a factor a rear mount turbo for $4,400 would be the way to go.
I've played the turbo game and for regular traffic the lag doesn't make it appealing. The nitrous isn't a bad game but I want my power on tap all the time without having to refill, not to mention having to arm it, so you need to know ahead of time that you want to use the power. With superchargers the lag is less than the turbo if at all depending on the SC and there is no refill necessary. I am looking for power to use at the drop of the hat to get in and out of traffic.