There isn't a great deal to choose between them - Mandroid evil criminal synthesis of metal and flesh or Warmonger the greatest combat soldier now turned evil. The two of them are locked up on a prison platform a hundred miles above the earth's surface.
The idea of UCM (Ultimate Combat Mission) is that you are Mandroid or Warmonger, ie one of the bad guys and you want out. You choose to play one or the other at the beginning of the game depending on how human or droidy you feel. The other difference is in your equipment. The Droid has a slow but far firing laser the human has a fast action but short-ranged machine gun.
The actual game is not that astounding unfortunately. Small little sprites which trundle over a rather dull background.
It isn't rubbish - the programming is too good for that - but basically you'll fall asleep long before you get to the final screen.