I pick Zombies, because Zombies won't have any direct impact on the Environmental stability of the planet (emphasis on -direct- impact, as things like city fires or nuclear meltdowns caused by no-one being around to stop them would have immense impacts.)
Thing is, with zombies, you can survive off the land, in the event of a global Ice Age or a... what would you call it, a Desert Age? Thousands, probably millions, would die simply because it'd be impossible to feed them. You can't grow crops in the snow, and the power requirements to grow enough food for a colony indoors would be astromical once you factor artifical lighting, and heating to keep the crops growing on a yearly cycle. Perhaps there are ways around it, but still, Zombies are cooler anyway.