So why does MB sell whatever 320 higher than S80? The short answer is because they can. They just have a different demand curve than Volvo.
In terms of differences, the biggest one is that MB mounts the engine front/back driving the rear wheels first, while Volvo mounts the engine left/right driving the front wheels first. In general, front/back/rear set-ups are thought to lead to better handling, but you may or may not care about that... Only a test drive will tell.