My initial reaction to this question is: which world would inevitably be better for me? Both fantasy and science fiction worlds have their good and bad points (for the sake of argument, I'm going to stick with the stereotypical examples of these genres).
Fantasy worlds have magic, fantastic creatures, swords, talking animals, fair maidens, noble kings, and other such goodies. But fantasy worlds also have evil overlords, evil creatures, wars, bad food of the bland variety, and prophecies that always seem to put the world in chaos for a short period before some "chosen one" can come along and put things right--why it is more rare that fantasy characters don't get ticked off about this last one doesn't make much sense to me.
Science fiction worlds have spaceships, alien races, faster-than-light travel, cool gadgets, space adventures, and damn good food. But science fiction worlds also have interstellar wars, alien invasions, evil aliens that can't talk, but manage to wreak havoc on ship crews, moral quandaries, technology that goes bad, and sterile environments or excessively dirty environments.
Which is better? Do I want magic and unicorns or spaceships and aliens with three heads?
On the one hand, perhaps I could use magic to escape from all the darkness of a fantasy world, but that assumes that if I lived in such a world I would be fortunate enough to have magic. This also makes me a terrible coward. A fantasy world also means I can't take all the things I currently enjoy with me. No computers, no Twitter, no WISB, no nothing. A science fiction world means I can keep my gadgets, but I also would have to face dystopian societies, interstellar wars, space pirates, and the like.
Do you see the dilemma? In some ways, I think science fiction worlds are harsher than fantasy worlds. Sure, in science fiction worlds a lot of things are handed to the characters: they rarely have to worry about food or water or running out of laser guns or whatever. But, these same characters live in a world (or galaxy, I imagine) where governments turn into Big Brother, robots reject and kill their masters (a simple snap of the neck will suffice), interstellar wars rage for decades, decimating planet after planet, civilization after civilization.
That's not the say that fantasy worlds are a picnic. At least in science fiction worlds death is quick (usually). Laser guns tend to kill people a lot faster and less painfully than bleeding to death on a battlefield. Characters in fantasy worlds also don't have the luxury of food processors or good storage methods (no fridges in Narnia, I'm afraid). Plenty of salted meat, moldy fruits and vegetables, and food poisoning. Plus, the "chosen one" is almost always some stupid farm boy who takes forever to fulfill prophecy, which is unfortunate if you're impatient like me. While fantasy characters are waiting for someone to rescue them, evil warlords are turning them into slaves or outright killing them in droves (I suppose similar happens in science fiction worlds).
But a fantasy world allows me to do things that can't be done in a science fiction world, primarily because science fiction worlds should adhere to standard rules of science. I've always wanted to shoot fireballs out of my hands or take a trip on the back of a dragon. Having my own set of armor and an enchanted sword would be pretty cool too. Or, I could be a bard and travel the world telling amazing stories about people that lived or never lived, captivating the minds of children and adults alike! When I think about fantasy worlds that way, it certainly makes them much more appealing.
How do I decide, though? Would I be willing to give up Twitter or computers or the possibility of travelling into space? Would I be willing to give up dragons or magic fireballs or enchanted swords? Which one would be the best fit?
If I had to take a guess, I would say that the science fiction world would fit best, but only because I can see how difficult it would be for me to give up technology. Technology is as much a part of my everyday life as breathing and I can't imagine being able to toss that away without thinking long and hard on it, and even then I would be apprehensive and might even regret going to a fantasy world. But I might regret going to a science fiction world too. It all depends how it turns out. Would I end up in a good fantasy/science fiction world or a bad one? Perhaps if I knew that information ahead of time it would make the decision easier. I'd be much more likely to forego science fiction for fantasy if I knew that the fantasy world would inevitably be a better place for me, one where I wasn't going to end up enslaved by evil three-eyed squid-like aliens.
I guess I'm still on the fence.
What about you? For the sake of making this argument easier, let's assume that you could choose between a perfect fantasy world or a perfect science fiction world. Which one would you choose?