Now what ever indicated that Penny was a prototype? For all we know she is the result of testing from prototypes, which presumably is what Gren is. As I've mentioned before, while Gren is not primarily robotic he is a cyborg, not an android. He was once a faunas and simply started replacing parts of his body after getting more and more injured. Penny on the otherhand is fully synthetic i.e. was never born conventionally.

The main difference is most notable with the machine that helps Gren work his aura. On it's own, what he uses isn't enough to produce aura, so he feeds it his own so that his aura can be used throughout his body. Conversely, whatever Penny is using is powerful enough to produce it's own aura, instead of being fed by an already existing aura. Not to mention that even if we were to use what you're suggesting, Gren still would be actively pumping aura through his body the same way he would be actively breathing; it's just something he'd always be doing. More so since Gren actually relies on his aura to power his body. The only reason he eats and such is both for the taste and to create biofuel for his thrusters, which isn't necessary for him to function anyways.

Of course, then we'd have to debate what is more effective armor: Aura or one and a half inch thick armored flesh.