As an author, I have written both fantasy and science fiction. I think Science fiction is much harder, since it has to be bound to the laws of the universe in which the writing is set, whereas fantasy writing can go places beyond the limits of the laws of the universe for the book setting.

I love to write fantasy, because it's limited only by imagination (though I'm not a fantasy writer; I just enjoy writing it sometimes.) Science fiction, on the other hand, requires some research and the sci-fi fans are brutally tough on a new sci-fi author.

Obviously SciFi would be harder. Because of the "Science" bit.

As a young writer.. I haven't explored much writing in a sci fi novel but for me it is harder to write a great fantasy novel. In a sci fi novel, of course it is hard because there are certain laws of the universe wahwahwah or anything but atleast you have this as a guide when writing your novel.

For me, writing a great fantasy is harder because a FANTASY has NO LIMITATIONS. it has endless possibilities. hmm for example you can say the pigs that fly are aliens and they are spies from venus to spy on us here on earth. hmm makes no sense right? but it doesn't need to be justified or criticized because there is no NONSENSE in fantasy..you can make anything your own. For example, write a sci fi novel in space and say that for example KABOOM! it made an explosion so loud that all the people in the earth became deaf (LOL sorry i suck in examples haha) but there are flaws in here and make it harder for the reader to believe this because there is no sound in space. somehow you still have a GUIDE on what to write on.. because it has LIMITATIONS..lol..do you get me here? I'm kinda lost myself but I hope you get my point.

for me writing with limitations is easier because it creates a clearer pictures while a fantasy novel has no limitations and alllll ideas are scattered all over the place. of course I haven't written a sci fi yet, and this is just an opinion :)

Writing a purely original fantasy is rare this days because it is hard to write a fantasy novel that no one has ever seen before.. for example, TWILIGHT or HARRY POTTER are great fantasy novels but they are not purely original...Harry Potter is a mixture of witchcraft and magicians and so on while twilight (sorry overrated examples) are from previous vampire novels..

lol..i think im getting lost again.. back to the point, the fact that it is very hard to find a purely original fantasy or fantasy that is unpredictable (if you have read a lot of books already sometimes it becomes easier to predict the ending) means that it is harder to write than scifi novels. There are many laws in science you get ideas from but in fantasy.. YOU ARE ON YOUR OWN...