OK, since this has gotten so out of hand before I could say anything, I'm going to pretend that the last page or so doesn't exist.
The designs as they exist right now are too simplistic. The red ones just need detail in a general sense, make the top more articulate, provide some interesting texture, and pretty much doing whatever adds to the semi-organic form. They look like a claw of some kind, which can potentially be good. The white ones, on the other hand, need a good bit of work. I have nothing against a rectangularly-shaped ship, there is nothing in space that really requires aerodynamics. That said, it still needs more variety than a simple rectanglular prism in its fundamental shape; perhaps a depression running around its circumference in one direction, or some splits in the armor plating that let techy stuff peek out (perhaps a good place for guns, if you can arrange it that way), armor plates period (i.e. modelled, not textured), more breaks on the large surfaces to distract your eye from the regularity, and lots and lots of small greebles. Still art will be held up to much higher levels of scrutany than moving media, so you've got to pour on the detail. Book art is no place for the low-poly model, I'm afraid, and right now that's exactly what you have.