240 flywheel.
in 2 conversions and over 50 kmi of driving I havent seen any ill effects (even w/o re-balancing)
Actually my first conversion was with a flywheel from a 220 D , they are much heavier than the 240 D flywheels (as are the 4-spds from them) and that did have some vibration, but the 240 one has none.