It depends on the exact type of zombies. Are they the usual shambling ones, or super aggressive runners like in Left for Dead or World War Z? Does the contagion spread only through bites, or is it airborne? Maybe they could be the type of supernatural zombies that don't just fall apart after a while but last forever and can walk between continents through the deep ocean, even if it takes them a long time?

Of course, alien invasion would depend on the type of aliens too. Are they aliens who just discovered something we never did, took a road we never did, and so they can travel between the stars, but they do it in basically 17th century galleons while fighting with flintlock guns, like in this incredible story? (PDF warning) And then they land on Earth, thinking they'll conquer it just like a myriad other worlds, and get assfucked by supersonic jets the and Marines using automatic rifles? Or maybe they're like the aliens in Mars Attacks!, and you can defeat them by playing folk music? Or maybe they're like the aliens in The Three Body Problem, and (spoiler alert) we're just fucked with absolutely no chance to fight back in any meaningful way?