[nasredin]

For me, the distinction between sf (the science fiction fans call it sf and reserve the term sci-fi for the mainstream junk production full of bad cliches etc.) and fantasy is rather clear:

In a fantasy setting, the civilization usually fades away. The Ancients possessed the great knowledge that is mostly lost. The power belongs to those who manage to find an archaic spellbook or a buried amulet and learn it's secrets. The wizards teach the secrets of their art to their apprentices, but as something inevitably gets forgotten, each subsequent generation knows less and less.

All that has direct relation to the situation in mediaval Europe, rich with Roman and Greek artifacts.

On the other hand, in a sf setting, the civilization is on the up grade. Every day brings new scientific discoveries, technologies and devices. Any ancient 'artifacts' are obviously useless old junk or some historic curiosities to be put into museum at best. The knowledge becomes obsolete extremely fast and the yesterday's apprentices work on the things that their tutors can't comprehend.

Not a surprise, rocket science is one of the most popular sf themes.

Finally, a good writer just writes quality literature that doesn't necessarily fit into a particular genre. I've read books that neatly combine magic and technology.