I've finished Jacob de Zoet and coming to the end of Ghostwritten, so here are some more thoughts on both.
First, Ghostwritten is the more obviously bravura performance. I loved Jacob deZoet, but parts of Ghostwritten took my breath away. But Mitchell is playing post-modernist tricks in deZoet as well. The slave narrative at the beginning of part III and the long poem at the start of chapter 39 are the most obvious examples, but I wonder whether the whole of part II is another. I think part II is the weakest section, with all of hugger-mugger straight out of a Gothic novel (evil monks, crazy death cults, an inescapable monastery), but I wonder to what extent readers are expected to realize that it's a deliberate borrowing, not necessarily to be taken seriously. For that matter, I wonder to what extent a reader should take it seriously at all -- it feels pretty intrusive into the otherwise-realistic flow of the novel.
Second, Mitchell has an amazing ability to inhabit his characters and give us their voices. Again, Ghostwritten is the more obviously virtuosic of the two, but I think that creating a pious man like Jacob without making him a goody-two-shoes is a real achievement, as is the creation of Penhaligon. To me, this is the major thing connecting the two novels -- Mitchell's amazing array of voices.
Third, I'm now a David Mitchell fanboy, and I'm sure I'll be reading his other books in the near future.