Amateur opinion here. I like the above designs but they do read as armor variations for the same character. Might be a stronger difference if it was in color and you had different palettes. To me, concept art is visual problem solving with each entry being a mostly unique attempt at getting close to your creative objective. It's about choice. If I was on the other end and had to chose, I wouldn't feel like I was really getting a choice. Do you prefer the red mini-cooper with the cream roof or the green mini-cooper with the black roof? Your overall designs for these guys don't deviate much from each other. I do however like the contrast changes you made, that definitely helped quite a bit. You've clearly got the skills to do this. I believe that you can go all the way.