Here's Kij and here's Jo.
I write both, fantasy and science fiction, and there is absolutely no difference in my process between the two. I do think there's an air of elitism around science fiction these days. For one thing, it doesn't sell as well as fantasy, so it must be more erudite, natch.
However, to argue that fantasy is y (backward-looking, or about 'chosen ones' and internal resources) whereas science fiction is x (forward looking, or concerned with intellectual achievement) ignores vast swaths of both ends of the genre. And the simplistic argument, which is to say that a science fiction novel that subscribes to the chosen-one plotline is fantasy wearing a veneer of science fiction (Dune, anyone? Lensmen? I could go on--) is a logical fallacy, the serpent eating its own tail. "I say that all things of the set including these characteristics are Y: therefore, this thing, although it purports to be Z, is really Y because it includes the characteristics of set Y." Um, no.
That's as problematic as saying that fantasy novels must be "comforting" or end with the created world reset to some ideal, idyllic state, or be backwards-looking. In which case, I think Swanwick's Jack Faust and China Miéville's The Scar are science fiction novels.
I know China's work is purported to fall into a category between SF and Fantasy. That's my point, I think: it's all, to my eye, different angles on the same set of literary techniques.
I actually think fantasy and science fiction are a continuum, which is why I prefer the term speculative fiction. Not because I'm all hoity (I'm all hoity for other reasons), but because I think the division between SF and fantasy is very easy to make: If you call your science magic, you're writing fantasy. If you call your magic science (remember those orbital web-spinning spiders? how about John Varley's Titan books?) then you're writing science fiction.
If you woddle around between, you're writing spec fic.
I'm a lumper, not a splitter. Does it show?