I'm just about the spaciest space-nutter around, but why the hell are they spending precious money and opportunity to fly a freaking demonstration instead of another actual observational tool?
Look, we know the composition of Mars' atmosphere. We know how much sunlight falls there, what the temperature range is, and so on. It's dead simple to set up a testbed here on Earth, in a jar, and run the oxygen-production process in the testbed. Better yet, you get to measure its output, tweak its operating parameters, and even do an autopsy on it if something goes wrong.
The only thing I can see us getting out of "make oxygen just like we did before, but ON MARS" is PR, and I don't really see the PR upside. All the science packages that were accepted, and a lot of them that didn't make the cut, would've given us new knowledge about the planet. Why in either world are we sending this package instead?