Its not quite the question you asked, but I recently changed from a 156 SW to a GT. Bearing in mind the 159 is slightly more roomy than a 156, I noticed the following things.
There is more leg room in the back of an SW, but not much. But what my rear seat passengers have told me, the SW had more head room for them, plus being a 4 door car it is much more practical for those in the back.
Also although the SW boot is larger, most of that space comes from the fact that the rear of the car doesn't slope down anywhere near as much as the GT (hence the head room comment above). The actual boot space under the parcel shelf is pretty much the same. You could stack stuff much higher in an SW.
But, importantly, all the time I owned the SW, I always had this nagging feeling that it was just an Estate (a bloody good looking one mind). When I switched to the GT I felt like I was in a car with much more presense, (if you get what I mean).
Hope that helps (not sure it does though