I can’t say what they were going for, but the way I take it is that in ROTS, the helmet is new and perfect. By the time 19 years pass, Vader has been active, doing things. From what we see of how things go for him in the PT, he likely has done plenty to get his armor and helmet all beat up, maybe even replaced, a few times. Maybe in ANH, things have been too quite for a while and his armor is looking worn and then when he comes back in TESB, it is freshened up. A single outfit is not going to last someone like Vader for 23 years without some maintenance, upkeep, and the occasional replacement. I think making the outfit look new and pristine in ROTS was a logical choice. No, it doesn’t look as menacing as in the OT, but it isn’t supposed to.