Coincidently I was listening to an interview with Carlos Huante yesterday or the day before where some of the designers on Prometheus were noting the way their designs on Prometheus for the original Engineers script, had a number of specifics. Especially the main monster that came from the Sexburster - the Porpoise-headed one with the whiteish skin... I think Co. Hicks and I were speculating that we wouldnt be surprised to see that scene and creature in this film. So in the interview I listened to, the designers were stating that that creature was going to have a very easily seem human skull under a thin layer of transparent skin.
That descriptiom fits with both of these designs.
Honestly, even if some of the designs that made the cutting room floor on Prometheus were better than what made it to the final film, i never liked those designs as much as these. This is a good sign.