Science Fiction vs Fantasy

I've always preferred Science Fiction to Fantasy. I recently ran across this entry from John Scalzi's blog defining the difference between SF and F:

Call me unbearably shallow, but here's how you know the difference. You walk up to the main character of the story in question and say: "Hey! Main character! That deus ex machina doodad you have on your belt, does it have, like, a battery?"

If he says "Why, yes, there's a tiny nuclear fuel cell in there that will power this baby for 10,000 planetary revolutions," well, then, you've got some science fiction there. If he says, "Of course not, it was forged in the eternal flames of Mount [insert typewriter spasm here] by the dwarves who serve the elder and/or fallen god [insert second typewriter spasm here], and holds captive his immortal soul" or some such, well, that's fantasy. Everything else is pretty much elaboration and variation on the point.

It seems like there is not that much difference after all. However, I've always been a fan of SF ever since reading "Have Spacesuit Will Travel" by Heinlein. To me, SF is more the "What if civilization took this path because of one or more futuristic events." Where Fantasy tends to morality tales of epic proportions. One of these I liked was the "Thomas Covenant Chronicles" by Stephen Donaldson. Firefly/Serenity fills both squares.