That depends on what you expect in terms of performance and what you are willing to put up with. I have the light aluminum Comptech flywheel and I am very happy with it. Keep in mind the purpose of a flywheel, however. If you go lighter, the car will be a bit 'touchier' at low speeds and you will have to learn to engage the clutch more carefully at low speeds. That said, I only had to drive mine for a day or two before it became second nature and I did not notice the difference any more.
Also, a lighter flywheel will theoretically make your car a bit slower on the initial launch from a stop, but it will accelerate quicker once you are rolling at all. It is a nice improvement in those situations where you are just rolling along with traffic, then punch it to pass someone.