Overall, I prefer science fiction to fantasy.
One reason I prefer science fiction, although I love both genres, is because (generalizing wildly) science fiction can (more easily) incorporate aspects of fantasy, but fantasy can’t (as easily) incorporate aspects of science fiction.
But if I poke at that reason, it starts to fall apart. So maybe it’s just the intriguing aliens and the unexplored frontiers and the mysteries of the future—not the hardware so much as the heart of science fiction—that satisfies me. I grew up during the Kennedy administration, I wanted to be an astronaut, and I believed the dream of shiny space ships going to the stars. Tiptree’s “The Only Neat Thing to Do” made me cry.
Nowadays, when I write novels, I write science fiction. When I write short stories, I tend to write fantasy. I’m not sure why that is, but I suspect it is because science fiction sustains me, and I can sustain it for the length of a novel, whereas fantasy comes in flashes of inspiration that would fade away under lengthy scrutiny.
Science fiction is for building on.