Kat has written a wonderful post about the question of the difference between fantasy and SF, and why some people like one and not the other.
I’ll throw in my own two cents: I think these kinds of fiction speak to different areas of the imagination. I started out as a reader and writer of fantasy, simply because I loved imagined worlds; the more complete an imagined world, the more I loved it. (I’m sure this was at least in part fueled by my fascination with the Forgotten Realms line of products from TSR. I regard it now as a rather mediocre fantasy world, but back then it was, for me, an example of collective worldbuilding gone mad.)
Later, I felt that fantasy was too dislocated from the things in our own world that mattered most to me; fantasy was a kind of PG-13 filter on the world, where swords and the victory of the good replaced, you know, crappy political and economic situations. In my late teens and early adulthood, I began to feel that horror more honestly reflected the human psyche. So I got into Lovecraft, Straub, and the highly literate Borderlands books edited by Thomas Monteleone.
But you know, there’s something missing in horror, too. For all the light, horror came to represent to me a kind of arrested psychological development: it focused on the frightening, the scary. Fantasy contained it, and horror glorified it.
What brought me to hard SF, then, was a desire to confront the real world — or something very much like it, only a few steps removed from our own world — and a desire to see some balance between the frightening things in the universe, in whatever form they are presented, and the possibility that human ingenuity could ensure survival or even triumph.
But the thing is, none of these genres represent real things to me. China Miéville straddles all of them (if we count steampunk as SF), though he weighs in a bit more heavily in fantasy and horror. And Miéville appeals to me for just that reason: he isn’t bounded by genre, yet he plays freely at mixing and matching elements of different genres.
This fits well with my musical aesthetic, and my earlier approach to musical composition, and it also fits well with my approach to all kinds of things in life. Still, in general I like the boundaries and freedoms allowed me by mainstream hard SF better than any other style of writing I’ve tried, aside from poetry. SF, for now, is my serialism or set theory (to use the musical compositional terms): it’s a set of self-imposed restrictions I embrace because they allow me to write more interesting things explicitly because of the self-restrictions — the restrictions themselves are, for now, freeing.
But I would no more tie myself to a single genre, or a single conception of a genre, than I would tie myself to the mast of a single ship for life. Living people don’t work this way. A thinking person can change his mind, change direction, and surprise himself. If you cannot do that, there is no point in writing at all.