Just going by the sprites as they appear. The replacement is a more contrasted color with the backgrounds of the base, it's a little more refined, and I think the scale is a little better. The Big Chap sprites make em look twice as big as a Predator in certain situations, also it seems a little more jagged and the monochrome color blends into some of the base walls a little more.
As to the model mystery that still leaves the Predator. The Halcyon Predator 2 kit does not have a mask. Though they could still have used it as a base, I guess.