Lighter flywheels are generally a good idea. Most car manufacturers design/build engines with street driveability in mind (ie for grandma's & chicks etc), which typically in this topic of conversation might mean a heavier flywheel than needed.

The downside to lighter flywheels generally, would mean the inertia or centrifical force generated would be less, and hence the car would de-accelerate faster when off the throttle. This has implications for hill driving and taking off.

Personally, i'd go a lighter flywheel, but nothing too excessive, as the same acceleration effect can be gained with gear box final drive modifications, without expending inertia.