I am seriously considering getting into RZ. As I look through KEH, I see there are lenses with designation "W" "ULD" and none at all. It is my understanding that one with no designation is the oldest, W is the next version, and ULD is the best one for this platform. Can someone give me some prospective of what these differences really mean in real use? I see plenty of "W" types are available with good price point, and ULD are fairly expensive and not as plentiful. Opinions from folks who actually own these lenses and use them on regular basis would be really great.