Depends on your application. If you have the power (boost, nitrous) you can benefit from a lighter flywheel because the inertia won't be there on the launch, but you will still have the power to carry it through. If you are using it on an n/a deal, make sure you got enough power.
A lighter flywheel will not shock the tires as hard.
So, if I were looking to run 17" drag radials at the track (which are notorious for not hooking with an M6), would it be better to go with a lighter flywheel? I'm only pushing 400-425 to the wheels (est.) but I do plan on spraying 150 HP to it in the future. Just trying to decide which flywheel to go with when I order my clutch.
I think that you would be fine based on the tire mentioned and the plan for a little nitrous in the future. It is important to remember that more often than not that chromoly flywheels are cut very thin to reduce weight. As a result that have a greater tendency to warp as a result of heat that a similarly built steel or even aluminum flywheel. This isn't to say that chromoly is bad...you just need to insure that it isn't cut so thin that warpage will be an issue. I hope this info helps. Let me know if you need anything else. Thanks,