I've gotta say sci-fi. But thats mostly on principle.
I find with fantasy it leaves so much room, allowing wonderful and amazing storylines and worlds that are so diverse.
But then, I've always preferred sci-fi, why? Not really sure, I'm just drawn to the idea of the future, and what it holds for humanity.
Still, what I enjoy about most fantasies is the wealth of detail that can accumulate behind it, like in Middle Earth. Creation myths, old legends and so on.