Hope to clear things up for you.
The Revision C was prone to splitting and has been replaced by the G revision. The early G revisions had a few that split but they released a newer G revision that is more robust and so far none that I know of have failed.
When the C revisions were failing people (including myself) went to the D as it was the best solution on the market at the time. Some went the Forge DV route but for a simple plug and play the D was the best. Now the D revsions have been known to leak, as was proved by RTech Nick. The end of the piston does not have a rubber seal so air can leak by, whereas the G revision has a seal on the end and makes a better seal. The G is designed for both the K03 and K04 GTi variants. The D is from the Audi S3.
Just to confuse you the D has now been replaced by a new C revsion, which is basically a D without the cage around the piston.
Which one to chose will be up to you, as people swear by which ever one they have fitted. Having had the D and G fitted I prefer the G, as the car runs smoother and feels more powerful throughout the rev range. Plus its cheaper by quite a bit and cars running 340bhp are running the valve without any issue.
Hope this makes it a bit clearer for you
