Out of what i've read here on the forum the difference is very little between the M40 singlemass flywheel and the twinmass M42 flywheel.
The M20 flywheel has another advantage except the weight: The M40/M42 flywheels have 215mm diameter and the M20 one has 228mm diameter => less power loss with the M20 one. :rolleyes: So i'm still gonna go for the M20 setup.