Sometimes one encounters fans of speculative fiction who love Isaac Asimov but hate J.R.R. Tolkien; who like Stephen Donaldson but can’t abide Arthur C. Clarke; and so forth. This thread is aimed at people like that: readers who have a strong affinity for science fiction but cannot stand fantasy, or the reverse. Why do you feel this way? Can you point to any particular authors whose work encapsulates your reasons? Are there any exceptions?
I prefer fantasy usually. Generally, I find most science fiction of the Asimov type too dry. I’m more interested in adventurous/romantic stories than I am in how (IMO) nit-picky details about how spaceships/robots/alien societies work.
Can’t help you with the way you’ve worded the OP. I *do *have a strong preference for SF over (non-Pratchett) fantasy, but it’s not overweening the way you seem to be looking for.
But as an info point, I tend to find a lot more standalone books, and politically, more anarchist/socialist and feminist works in SF, so that’s why I have a strong preference.
Banks and Le Guin represent everything I love about SF.
But I love Tolkien, Gaiman, Moorcock and Pratchett, so I’m not *completely *averse to Fantasy. And Miéville often straddles the line quite nicely.
I don’t think there’s any difference. Science fiction IS fantasy.
I tend to read more books labeled “fantasy” these days, mostly because current SF has become far too limited and lacks imagination. Fantasy offers far more options these days.
I like both, but I tend to prefer science fiction just because while I enjoy really good fantasy, there just isn’t as much really good fantasy as there is really good science fiction, in my opinion. Whereas there’s a LOT of really bad fantasy. Hard to separate the wheat from the chaff.
Also I tend to find fantasy more subjective - I agree with most (MOST, I don’t get why people like Ringworld and Foundation, for example) of those “top 50 sci fi books” but tend to agree with less of the selections on similar fantasy lists. On the other hand I’m rereading Barbara Hambly’s straight fantasy books and loving them.
What, exactly, do you mean by fantasy? Do you make any difference between the Fantastic or to give it a name I’m more used to, a) the littérature fantastique, and b) the Tales of the Grotesque and Arabesque as Poe put it and c) the sword & sorcery fantasy?
How would you define Kafka, for instance, and the meta-textual work of Borges?
And where do you make the cut between fantasy or, more general, the Fantastic and Science Fiction?
I prefer science fiction to fantasy, but I have (and will again) read fantasy novels and enjoyed them. I can’t really say why I prefer one to the other; maybe fantasy seems more about the relationships and less about the science and the whole “what if THIS weird thing were actually true?”, which is what I love about science fiction.
ETA: My favourite genre, mind you, is post-apocalyptic fiction, which, you could argue, is a blend of both.
Of course, SR Donaldson has written science fiction as well…
SF lover, fantasy… “hater” is way too strong, but I don’t read it.
Anyway, I like sf because it is, however remotely, possible. Sf novelists try to keep their work grounded in scientific theory, whereas with fantasy, you’re just making shit up.
All imho, of course, and I’m sure there are examples and arguments that “disprove” my points, but I was asked why I have a preference, not to come up with a hard 'n fast definition that will apply perfectly for both genres. 
I come from this perspective, too. I love well-written stuff in any genre, but am much more open to guilty-pleasure sci-fi than I am to equivalent fantasy.
The Foundation Trilogy, Heinlein, the other Golden Agers, etc. - just felt different than LoTR and other big Fantasy titles. Neither was better; I just felt myself drawn more to sci-fi: sci-fi positioned its escapism as possible, however far out; fantasy asks that the Magic line get crossed, too.
Don’t bother me with details.
Why is keeping work grounded in scientific theory a virtue?
Also, I’d say that high fantasy’s seminal work, Lord of the Rings is very grounded in linguistics, or at least philology. It’s a story about language as much as anything else.
Oh, I have no problem with anybody profferring their opinion. Well, scratch that. I have no problem with opinions so long as they’re not in favor of Piers Anthony. 
Some fantasy IS just making shit up. But in the better kind of fantasy, the world/universe is self consistent. For instance, in Who Framed Roger Rabbit? (movie, not book), there’s a scene where Roger (a cartoon character) is wearing handcuffs, and Eddie (a live action human character) is trying to get them off of him. Eddie produces a hacksaw and begins sawing away at the cuff. The crate or table or whatever wobbles. Roger slips his hand out of the cuff and steadies it. Eddie angrily demands to know if Roger could have gotten out of the cuffs at any time, and Roger replies, more or less, that he could only do that if it was funny. See, the cartoon characters have extrahuman abilities…but they can only use those abilities under certain conditions. In a more classic fantasy, Barbara Hambly’s Dragonsbane, all magic must be paid for, one way or another. A mage can’t just blast a group of bandits when she’s attacked, because she’s simply not powerful enough as a mage. She doesn’t spend enough time meditating, because she’s also the lover of the local lord (mages aren’t allowed to get married) and the mother of his children, and she spends time BEING a lover and mother, rather than concentrating on her studies. Even if she spent all her time being a mage, though, her abilities are limited, both by innate talent and by the teaching she’s received. She has to make do with the abilities and training she has. And sometimes, that means that the best way to accomplish something is to use force, not magic. Now, I’ve certainly read enough stories that basically allowed a mage/psionicist to simply force magic to happen by sheer strength of will, and I consider this to be cheating. I don’t care HOW strong a wannabe warrior is, s/he’s not going to be able to fight effectively the first time s/he picks up a weapon. It takes training and practice. And bruises.
Cause I’m a forward-looking optimist - our world is awesome and it’s only going to get awesomer.
Science fiction uses science (or at least pseudoscience). You can argue that science fiction is a subset of fantasy, but I wouldn’t say fantasy equals sci-fi.
I like fantasy because I just like the idea of magic and things that are plain impossible. Much more interesting that way. It’s easier to suspend your disbelief for fantasy- if you have shape-shifters and you want to know where the extra mass goes, fantasy can just say “It’s magic” and that’s that. Sci-fi has to come up with an answer that’s somewhat probable.
Also, I like the themes that come with fantasy more than the sort you tend to find in sci-fi. For that matter (or maybe it’s the same matter) I prefer fantasy cliches to sci-fi ones. Prophecies of a chosen one with a magic sword and space colonists shooting aliens with laser guns are both overdone, but I find the ancient prophecies to be inherently interesting.
There are sci-fi books I have enjoyed, and I am planning to read Lois McMaster-Bujold’s Vorkosigan Saga, as soon as I can find all the books. But I don’t go specifically looking for it.
And that is exactly why I don’t prefer fantasy. If all you have to do is say “it’s magic”, then why not have a book like the following:
I don’t see how that follows.
I tend to read SF that deals with the future. For starters, that assumes that mankind has a future. And the futures you read about in SF tend to be better than the present… and the present is a helluva lot better than the past.
My love for science fiction is equal to my loathing for fantasy. I read science fiction because I believe most of it will be science fact one day, or supplanted proactively with something better.
The only chink in my armor is time travel fiction, because although I don’t believe time travel will ever be invented (because it hasn’t, to throw in a thought-provoking paradox), I am absolutely crazy for almost any story containing it.
Aside from the silliness of me being the good king, I can’t think of a single well-known fantasy story that goes remotely like that. Magic is rarely a story-breaker in published fantasy, if only because the greater magic tends to belong to the bad guys.
I prefer fantasy to scifi because I believe generally, scifi is more about the ideas at the expense of characters and the language of the writing. Scifi comes off as too dry sometimes.
Good “soft” science fiction certainly seems to be roughly the same type of thing to me as good fantasy. Le Guin’s Hainish stories (which I love) are mostly about people, not about science as such. The fact that those people are on other planets and sometimes ride spaceships isn’t so important; we could imagine more traditionally “fantasy” devices to accomplish the same narrative purposes.