That one with his hands behind his back is the one I've got, however he's probably a bit short.
I'm not sure exactly how tall Prowse is, or more importantly how tall he was when he made the films, but I saw one place that said Vader was 6 ft 7 inches. I'm assuming that's with the helmet and wearing boots. Mine's a little shy of that but easily remedied.
I also didn't leave the arms in that pose, but instead refiberglassed into a different pose.
Also, if you look closely, that mannequin's head and neck are cocked slightly to one side. More noticeable in person, and it also threw the armor off just a bit. I wanted the helmet centered, so I basically cut off maybe a fourth off the side of the head and neck (left side as you look at it). This allowed the helmet to be more centered.
And yes, it really doesn't matter if the entire head is intact since it doesn't show.