Vader the character has a specific height. Vader the character does not have a lightsaber with a tab sticking out to hold a broomstick. C3PO the character is a robot who is made of metal. C3PO the character does not have a man inside a skin made of fiberglass.
Because I'm not into props repros and behind the scenes stuff, a Vader reveal head should look like what we got from Sideshow, not like David Prowse. I want my collectibles to represent what the film makers illusion, what we saw on the screen, not what it took in the real world to create the magic. Vader (and others) are characters, not costumes.
Some people go nuts over Stormtrooper differences from movie to movie - that's all prop/production business. The Empire didn't issue replacement armor for all their troops during the saga. I'm fine with a trooper that looks screen accurate for any of the movies or even if it's an amalgamation if that's what it takes to get best of breed.
I trust the difference is obvious now.