I've decided to side with the alien story. I feel that, right now, a return to my science fiction roots is probably the smartest thing to do. It's me returning to a genre that I love, a genre that I don't think I ever really gave a strong enough chance.
No, I haven't abandoned thoughts of writing zombie fiction entirely. When you're a fan of multiple genres - science fiction, zombies, romance, undead erotica - you want to give each one a shot at least once, but there are only so many hours in the day, so many days in the week, and you've got to prioritize.