The diaphragm on a G seals better than the piston on the D
BUT...
because of differences in the location of DV in relation to the turbo between K03 and K04 engined cars, a diaphragm DV is prone to heat drying out the rubber and cracking on a K03, so this is why the D piston type is better on a K03 car. On a K04 car this problem doesn't exist so a G is therefore better.