Well, a couple things ... first off, Peter's not a candidate ... he's done with the class and is a DM. Part of why he posted is that when he was a candidate, the gear he chooses to wear was OK ... now, suddenly it's not. He's asking why? I think that's a good question that deserves a better answer than the one he's received.
Second thing is that there's a difference between being the student and being a dive leader. Every agency makes specific requirements about what students wear ... and different specific requirements about what dive leaders wear.
Third thing (OK, I just thought of a third thing) ... is that the agency determines what gear is requisite for its students. NAUI does, PADI does, and GUE does. The difference, in this case, is that you are asking what's the difference between PADI and GUE. The answer should be obvious ... and it's probably not appropriate to go there in this particular discussion ... :no
... Bob (Grateful Diver)