The other theory is there are infinite universes somehow being created all the time by every choice. Which to my mind isn't very Occam's Razor-ish. But it does exist as a theory.
You mean the Many-Worlds interpretation? That's not every choice, but every quantum measurement. And it's not necessarily an infinite number. And it's not a theory, but an interpretation. And it's actually very Occam's Razor-ish (at least that's what proponents claim), because it needs less postulates than the standard Copenhagen interpretation. The many worlds are a consequence, not a postulate.