The dual mass flywheel is basically two flywheels stuck together with a bunch of springs inside and its designed for the engine to act as a damper at idle and when load is placed on the crank ie A/C.
Here's some info about the lightweight flywheel:
Aftermarket lightweight flywheels are designed to weigh less than the SMF and the DMF by at least 50 percent. The most common ALFs are constructed either of aluminum or steel. Most aluminum flywheels feature a replaceable friction surface, making them easy to rebuild. On the other hand, a steel flywheel can be resurfaced only once.
Pros: On a naturally aspirated vehicle, an ALF is a good way to reduce excess rotating mass, allowing for increased acceleration.
Cons: Excessive low end clutch chatter, premature synchromesh wear, and possible engine damage will occur if the ALF is too light or not properly balanced. If the ALF is too light, a lack of initial inertia can cause the vehicle to stall easily when moving off from a standstill.