The thing about the turbo 4 is that it's a simpler, easier, and less expensive motor to work on and there's a *LOT* more of them out there so parts are easy to find.
I've got almost 270k on my turbo, quite a bit of play in the shaft but not leaking or rubbing yet. Volvo turbos are no different than any other turbos, my 740 has a Mitsubishi turbo while the 240 has a Garrett. They can be rebuilt for a few hundred dollars or you can buy the whole cartridge for around $650-$800, so it's not a cheap job but it's very rare for them to fail in less than 100k miles and usually the water cooled ones will go more than 200k so it's not really a big issue.
The 960 has a great engine, but there's not any cheaper to run or maintain than a 4 cyl turbo.