Alien Invasion. Zombies no longer have reason so they will always come for you and the slightest scratch could turn you. Not to mention that the virus could eventually mutate into something much worse.
Alien Invasion would suggest an intelligent (or at least as smart as humans) race. Meaning that they may be open for reasoning or, worst case scenario, all out war where, like in Falling Skies, our only option would be to make the occupation as costly for the invaders as possible.
EDIT: However if Alien species has any sense they would just avoid us.
