I know a lot of the top tuners and specialists prefer the G - R-Tech being one of them.
It seems that between the D and G it's swings and roundabouts. The piston design of the D is in theory more reliable than a diaphram valve like the G over time, but still 'seeps' boost due to the design.
The G holds boost better, but is more prone to failing sooner as once the rubber splits it's bye bye boost.
K04 and K03 cars have the DV in different locations - I think on a K03 it's closer to the turbo, so the diaphram G may have a shorter lifespan than on a K04, due to the extra heat.
Either way, the best suggestion I've heard is to replace the DV with a Rev G regularly, either once a service or every other. They're cheap enough.