They differ quite a bit in many ways really. I love Pratchett's quirkiness, but I prefer Gaiman's writing style by a large margin. I'd recommend just getting American Gods or Neverwhere for the 7 bucks or whatever it is in paperback and reading them to find out for yourself, or just previewing them at amazon or whatever to find out. His books are more self-contained and darker than the light-hearted Pratchett's work, but that's not a bad thing. He's definitely a high quality writer and more than worth the time to look at, imo. If you want a good fantasy book and haven't at least tried him out, you're doing yourself a disservice