Depends on what I'm after. Pratchett is definitely the "funnier" of the two (although his footnote gimmick got old rather quickly), whereas Gaiman tends to pace things differently (read: faster). Gaiman for serious stuff - Neverwhere, American Gods, Stardust (read it even if you've seen the film) etc.
That said, I do enjoy both authors, but Pratchett is, maybe not weaker, but at least the one I personally get tired of quicker. If that makes any sense.