If I were at ground 0 of the infestation, Aliens would be worse. Aliens are just more dangerous by a long shot, and at ground 0, they are about as numerous as zombies. They win hands down. However, I would rather have Aliens infest a few towns over then zombies: Xenomorphs, once there discovered, won't spread as aggressively, and they are still living beings that need food, can be injured, have a certain gestation time, and their offspring are easier to quarantine once you know whats going on, and generally they provide a much clearer battlefront to which you can apply full military pressure. They won't spread as quickly or as surely, meaning if I'm a certain distance away, I'm much safer from the Aliens.