They both have good and bad examples. Fantasy used to be my thing, but I got bored of it quickly, as it is a lot of the same old stuff. Sci-fi I think has a lot more originality in it (at the moment), and thus I find generally to be a more interesting genre. I've been reading Iain M. Banks' Culture series recently, and I have to say it's brilliant. I'm a sci-fi nerd, lol
I'm a Lovecraft fan, and like Sci-fi/fantasy horror, there is somthing I find scary in the idea that some god-like race of creatures from outer-space are coming to wipe humanity out completly. The Great Old Ones are out there, and coming for us! I don't normaly care for Sci-Fi as in things like Star Wars and Star Trek, but I quite like Warhammer (bother the phantasy and 40,000) and am more a fantasy person, though I don't like to admit it. However, for me it's Horror that is king, and Pychological Horror like Jacob's Ladder, David Lynch, early Lovecraft (The Tomb and The Shadow Out of Space) and the video games Silent Hill that really does it for me. There is somthing about a story, in which the person is always guessing if the things they are going though are real or just them slipping into maddness that keeps me VERY interested.
I cant stand sci-fi personally. Spaceships and stars and all that just bores the **** out of me. Pass me a fantasy any day.
He writes both. For example, his Adept series is sci-fi, whereas his Xanth series, my favorite, is fantasy.