I should note that having a metal body does not necessarily add to longevity. My metal PRS-700 is probably on its last legs - not because it is not working, but because the bezel on the side - part of the metal case - is coming off. I expect if it comes too far off the reader will die... something will short out, or something else awful like that.
A solid sturdy plastic design could be much better than a poorly designed metal case.