I voted Gah!

It's just too difficult to pick one over the other. I've enjoyed too many good books in both genres to be able to absolutely say which trumps all.

However, that being said there does seem to be a surge recently of books that contain a fusion of both fantasy and science fiction. Two authors that can pull this off extremely well in my opinion are Neil Gaiman and China Meiville.

While Gaiman hasn't written a full length book that contains both genres, many of his short stories blend elements of sci-fi and fantasy wonderfully. Meiville is good. Really good. That man can describe snow in more ways than an eskimo. While the political messages in his books may leave something to be desired, I recommend all of his works to anyone who can appreciate deep novels where not everything is black and white.