Many have already chimed in, but I'm curious where you fall out on the "lumper" vs. "splitter" spectrum in terms of genre definition. At one end, we have the libraries, bookstores and publishers that lump all of it together under "science fiction." At the other end, we have folks that talk about highly specialized sub-genres as discreet entities.
There's some other muddying points: by fantasy and science fiction, do you mean traditional fantasy and hard science fiction, for instance? There are some works that really blur the boundaries -- Anne McCaffrey's Pern books are science fiction works with traditional fantasy trappings, while Star Wars or Dune are fantasy works with traditional science fiction trappings.
Or do you just mean, what do I do personally? I've been all over the spectrum in terms of trappings, although at heart, I tend to lean more toward science fiction. I only invoke the truly fantastic and unexplainable when there's a really compelling reason to do so, and I also tend to even regulate stuff like magic into a pseudo-scientific system.