Well, for a better image:
I’m glad you posted that comparison image actually as it just demonstrated for me another reason to keep the eyebrows and that is that they actually help make his face far more expressive.
Looking at left image I can actually recall and see the emotion he is displaying as he is telling Luke that he was right (that there was still good in him). Looking at the right image I couldn’t tell you what is going on from just this image as it’s a lot harder to read up front. I can tell that both images aren’t an extract frame comparison but I think the heavily contrasted eyebrows really help to convey Vader’s emotions in this scene where you can only really see his upper face, his mouth being obscured by the breathing apparatus.
.Val