Engineer Gills

Probably a difference in function. The sacrificial engineer's function is sacramental and therefore creates life. It's probably his only function. Hence he doesn't need to grow any biomechancal organs. The LV 223 engineers have a different function, I conjecture that their purposes have to do with terraforming planets- hence the need to be able to extract oxygen from terraformed atmospheres for brief periods. I also think they have a military function as well : to destroy those created lifeforms that don't make it to standards that know : "Sometimes you have to destroy before you can create." The gills also give them peculiar vocal patterns. Remember the deleted transmission scene from alien and how terrifyingly different it sounded? That's how the LV 223 engineers sound as well. When you see the movie again listen beneath the music as the holograms converse with one another. That's the scene where David first enters the cockpit. Well that's how they sound but much more subdued. We can infer that the Alien space jockey came from LV 223 before he landed on LV 426 for whatever reason. Best

can anyone post a pic comparison on this?

Just saw it again last night. There are definitely some gill patterns on the neck of the pilot.