Technology vs Magic
A world filled with movie characters with access to top shelf fictional technology vs a world filled with nothing but the most powerful magic users, items, entities and so on. Each planet is like Earth with a maximum population of 3 billion. Metaphysical entities such as Dr Manhattan exist on the Sci Fi Earth, while demons, gods and things like that exist on Magic Earth. Each side gets a year prep to conquer the other planet, the distance apart being about the distance between the real Earth and Mars.