Everything else equal, its just the receiver. If you were going to buy an M4 with a clear lower with no intent on changing it to metal, go with Velocity Arms' custom series. There is a video on someone taking a hammer to their polycarbonate body. It didn't break. Coat of paint and it'll do you well. Only gripe is that I learned their upper is plastic. Shouldn't be a problem if you're willing to do clear plastic lower anyway.
What's more important: morality or the freewill? In our society most people would say freewill. But we mustn't forget our morality.
Our morality guides our freewill to do the right thing.