Yeah, if you want a truly accurate ANH Vader, NEITHER is really accurate out of the box. It is MY OPINION, based solely on pictures we've seen so far, that the rogue one vader would be easier to customize into an accurate ANH Vader. This is largely because many of the nitpicky details appear to be better from the start on this version and thus require less work to get to the really accurate point.
It is also because of these better details (apparently accurate paint scheme on the facemask - specifically the eyelids, more accurate armor, better belt, shin guard straps, gloves, etc.) that for me personally, out of the box this guy might actually be "close enough" that I wouldn't even bother changing anything. Yes I can spot the differences between this and any of the actual OT versions of Vader. But unlike, say, the ROTS suit which is radically different in many ways, this version only varies in some more minor areas that varied from OT suit to OT suit anyway.
Please also keep in mind I'm specifically talking about this hot toys figure, NOT necessarily the actual suit used for filming rogue one!
EDIT:
For what it's worth, here's a list of what I think would be needed to make a truly awesome ANH style Vader out of this guy. Again, based SOLEY ON THE PICTURES WEVE SEEN:
-weather helmet and chest armor
-repaint shoulder bells
-lower dome (maybe)
-put tunic over chest armor (maybe replace tunic to get rid of pleats)
-get a ANH style saber hilt
-new cod piece
And that's about it. A lot less than what people have had to do to the actual ANH figure...
